Your PRI bill is a starting point. The amount you can actually stop paying, and the work required to replace it, determine whether a move makes financial sense.
Go to the total-cost worksheet or review the migration checklist.
What changes when you replace PRI with SIP trunking?
PRI and SIP trunking connect a business calling system to the public phone network through different interfaces. Replacing the connection does not necessarily mean replacing the PBX, its handsets, or its call flows. It does mean confirming that the retained equipment supports the proposed service.
A conventional North American T1 PRI has 23 bearer channels for calls and one signaling channel. Cisco's PRI documentation also describes configurations that share signaling across circuits, so check your actual setup rather than multiplying every circuit by a universal rule.
SIP trunking carries calls over an IP connection. The PBX may connect through native SIP capability, a session border controller (SBC), or an appropriate gateway. The supported design depends on the equipment and provider. For example, provider technical documentation calls for SIP-enabled infrastructure and sufficient internet bandwidth for peak calls.
| Decision | Retaining PRI | Moving to SIP trunking |
|---|---|---|
| Calling capacity | Check provisioned channels and current peak use | Confirm supported simultaneous calls and any separate inbound/outbound limits |
| Existing PBX | Confirm ongoing support and maintenance | Confirm model, software, licenses, and the supported carrier handoff |
| Connectivity | Keep the existing circuit and dependencies | Establish which IP connection carries calls and what happens if it fails |
| Numbers | Check numbers and services on the account | Confirm portability and a sequence for moving and testing them |
| Operations | Understand who supports today's service | Name the owner of the PBX, SBC/gateway, connection, and carrier faults |
Capacity and traffic are separate inputs. The busiest period determines how many simultaneous calls you must support; billable minutes and destinations influence usage charges. Neither follows reliably from employee count alone.
What does keeping your PRI service really cost?
Build the retention budget from your invoices, agreement, and renewal offer. Do not compare a discounted new-service quote with a historical bill that includes unrelated services or charges that will remain after the move.
Separate these items:
- Circuit rental, calling usage, numbers, and recurring provider charges.
- Equipment rental, PBX maintenance, and support that continue under PRI.
- Confirmed renewal changes during the comparison period.
- Necessary repairs or replacement work if you keep the current setup.
- Other services on the account that cannot be canceled with the PRI.
Review several representative bills if usage varies. Identify which charges disappear, which remain, and which change when the PRI is disconnected. If the same PBX maintenance cost continues under both options, show it on both sides or document its equal exclusion.
Past hardware spending is not automatically a future retention cost. Include what you will pay from the decision date onward, together with any relevant remaining obligations. Ask finance to confirm how it wants remaining leases and asset write-offs treated.
Which costs belong in the SIP migration budget?
Include the new recurring service and the work needed to make it usable. A provider's published calling rate is not a price for a fully configured PBX, a managed SBC, or a tested cutover.
Use the SIP trunking pricing comparison for billing models and provider-quote checks. For the migration decision, focus on costs that change when you switch:
- SIP service, numbers, emergency calling, usage, and selected support.
- Required PBX licensing, software changes, SBC/gateway equipment, or managed hosting.
- Any additional connectivity or backup needed for the agreed voice design.
- Configuration, testing, porting, and staff time.
- Old-service bills during migration and any enforceable termination obligations.
Inventory fax, alarms, elevators, paging, door systems, and other specialty devices separately. Some may use the PBX; others may sit on separate lines or the same billing account. Confirm each dependency before treating its cost as removable.
List main numbers, direct-inward-dial (DID) ranges, toll-free numbers, and numbers assigned to departments, applications, or fax. Record the carrier account and circuit each belongs to. Confirm what happens to existing PRI interface hardware, whether software or firmware must change, and whether provider-owned equipment must be returned. Check whether other sites or bundled services share the agreement before canceling anything.
If the PBX itself no longer meets your needs, compare a replacement cloud phone system as a separate option. Retaining unsuitable equipment to make the trunk quote look cheaper can defer rather than avoid a replacement cost.
Use this total-cost comparison worksheet
Choose a common decision date and a period long enough to include the contracts you are considering. Fill in both sides for equivalent traffic, numbers, support, and continuity requirements. An unquoted item is an unresolved cost, not zero.
| Item | Keep PRI | Move to SIP |
|---|---|---|
| Comparison period and actual service months | ___ | ___ |
| Recurring circuit/trunk/platform charges | ___ | ___ |
| Calling usage and overages | ___ | ___ |
| Numbers and emergency calling | ___ | ___ |
| PBX/SBC/gateway maintenance or rental | ___ | ___ |
| Connectivity, backup, and support | ___ | ___ |
| Applicable taxes and provider fees | ___ | ___ |
| One-time equipment, licenses, and configuration | ___ | ___ |
| Testing, cutover, porting, and internal labor | ___ | ___ |
| Old-service overlap and termination obligations | ___ | ___ |
| Confirmed credits, with eligibility and timing | ___ | ___ |
| Total over the comparison period | ___ | ___ |
For each option:
Period total = all recurring costs during the period + one-time costs + transition obligations − confirmed credits.
Use actual service months for each item. If the old PRI runs for part of the migration, include that period in the SIP option. If a charge is already included in another line, do not add it twice. Keep monthly rate changes and variable usage visible rather than forcing everything into one flat monthly figure.
A comparison period does not cancel a longer contract. Show commitments that remain beyond the period alongside the total.
How do you use the worksheet to make a decision?
Start with your current invoices, agreement, and retention offer. Use the proposed SIP service scope and written provider terms for the other column. Do not fill missing items with a market average or an assumed discount; request the missing information before treating either total as complete.
- Match the capacity, usage, numbers, technical design, and support requirements on both sides.
- Record each charge's source, whether it is recurring or one-time, and whether the amount is confirmed or conditional.
- Include migration work, old-service overlap, and contract obligations that the switch would trigger.
- Calculate both totals over the same period and disclose any remaining commitments beyond it.
- Identify what could change the result: usage, destinations, renewal terms, implementation scope, or an unresolved technical requirement.
Do not assume switching produces savings. If the SIP option costs less after all relevant items are confirmed, document the conditions behind that result. If an unquoted item could change the decision, keep the comparison open. A smaller monthly service charge does not establish a lower total cost.
When might switching fail to save money?
The move may cost more over your decision period if it requires substantial equipment or network work, if exit obligations are high, or if the existing service remains economical for the capacity and traffic you need.
A PBX near replacement can make a short-lived SIP adaptation poor value. A long overlap period can offset any operating-cost reduction shown in the proposals. Heavy traffic to destinations outside a bundle or quoted route can change the recurring total. A managed service can also cost more than a self-managed offer because someone else takes on work your IT team would otherwise perform.
Contract timing matters. Compare switching now with switching at renewal if both are feasible. Confirm any renewal notice window, auto-renewal terms, and commitments before deciding to wait. Do not assume a universal PRI retirement deadline applies to your carrier or location; get the provider's actual notice.
An option can still be worth choosing for supportability or continuity even if it is not the lowest-cost one. State that reason separately from the financial result.
What must be confirmed before you switch?
Financial approval should depend on a supported design and an agreed migration sequence. Confirm these points with the providers and the people who administer your phone system:
- The PBX model, software, SIP capability, required licenses, and supported gateway/SBC design.
- Busy-period capacity, number inventory, calling destinations, and any remaining specialty lines.
- Connectivity, voice-quality requirements, backup behavior, and local power dependencies.
- Number portability, account records, the port sequence, and testing responsibilities.
- Emergency calling and location handling for the applicable sites and users.
- Current contract obligations, provider-owned equipment, and who authorizes cancellation.
- Whether old and new services can operate together, how long that overlap is needed, and what happens if the cutover cannot complete.
- The named support owner at each handoff after migration.
The FCC's porting guidance warns against terminating existing service before starting new service and the porting process. Confirm the port and test the required calling paths before authorizing cancellation; do not assume the consumer guide's simple-port timing applies to a business migration.
Emergency calling needs its own review. The FCC's multi-line telephone-system guidance covers direct dialing, notification, and dispatchable-location requirements, including applicability conditions. Confirm what applies to your system and agree an appropriate testing procedure with the provider. This article is not legal advice; do not place uncoordinated test calls to 911.
Compare the provider paths before committing
Bring what you know about your PBX, current service, numbers, traffic, and renewal date. You do not need a completed worksheet to start. Discuss the requirement with SourceNet and evaluate available provider options. You choose whether to proceed; the selected provider confirms its scope, pricing, and contract terms and supplies the service.
Or review how SourceNet compares SIP trunking providers first.
See how SourceNet works. Confirm responsibilities and applicable commercial terms before proceeding.