SIP Trunking vs Hosted VoIP: Keep or Replace Your PBX?
Last checked: 12 October 2026
SIP trunking supplies internet calling to a phone system you manage; hosted VoIP puts the phone system in a provider's cloud. For an existing PBX, the useful question is whether to keep that system with new connectivity or replace it with a hosted service. Both can carry calls over the internet.
Our hosted VoIP guide explains the wider service. This comparison focuses on the decision facing a business that already has a phone system.
What are you buying?
Gamma describes SIP trunks as connecting a site's on-premises telephone system to its network over an internet connection. Your PBX remains part of the solution. Confirm its exact model, software version and supported connectivity with both the maintainer and trunk supplier.
BT describes hosted Digital Voice as a virtual phone system in the cloud. Its service supports desk phones and softphone apps, with package-dependent features. A hosted proposal still needs compatible user devices, a suitable connection and someone responsible for setup and support.
| Decision | SIP trunking with your PBX | Hosted VoIP |
|---|---|---|
| Where is the phone system? | Your existing PBX in this scenario | Provider's cloud platform |
| Existing equipment | Confirm PBX and handset compatibility | Confirm reuse or replacement of each device |
| Support proposal | Include PBX maintenance and trunk support | Include platform, devices and user support |
| Remote workers | Check the PBX's supported remote-user method | Check the proposed apps and licences |
| Cost comparison | Include connectivity and retained-system costs | Include users, features and device costs |
When is keeping the PBX worth investigating?
Ask for a SIP proposal if your system is still supported, staff are happy with it and you have a clear maintenance arrangement. Request written compatibility confirmation before treating retained hardware as a saving.
List any call recording, reception console, door entry, fax or specialist integrations. Have the installer demonstrate the required behaviour with the proposed service. The presence of a network socket on a device does not establish that it will work unchanged.
When is a hosted replacement worth investigating?
Ask for a hosted proposal if you want to change how staff take calls, simplify administration or replace ageing equipment. Demonstrate the actual workflow: a receptionist answers, transfers to a colleague at home, and sends an unanswered call to the correct mailbox.
Check which features are included for each user. Request a separate explanation for shared phones, reception users, mobile apps and any recording requirements. A generic feature list is less useful than a tested call flow.
A worked comparison for a 12-person office
Imagine an office with 12 staff and a supported PBX. Ask the SIP supplier to price the measured peak calling demand and all required PBX work. Ask the hosted supplier to price the 12 people's actual roles, including shared devices and remote working.
Build a comparison over the same contract term. Include setup, equipment, licences, calls, support, connectivity, backup and any outstanding equipment agreement. Do not compare a headline trunk charge with a complete hosted package. This is a comparison method, not a quoted price or a promise of savings.
Migration questions to put in writing
- Which current numbers will move, and who authorises the transfer?
- What happens to calls during the change, and who coordinates the suppliers?
- Which devices and integrations have been tested?
- Who owns a fault involving the PBX, router, connection or cloud service?
- What is the fallback when the office connection is unavailable?
- What power and emergency-calling arrangements must we provide?
- What commitments remain on the old equipment and service?
Include your IT provider in a trial of the proposed fallback. Our 4G backup guide and static IP guide provide useful questions for the connection supplier.
Frequently asked questions
Is SIP trunking the opposite of VoIP?
No. SIP trunking is a way to provide internet voice connectivity. The comparison here is between retaining your PBX and using a hosted phone system.
Will SIP trunks work with any existing PBX?
Do not assume so. Get the exact system and software checked and include any required configuration or replacement equipment in the quote.
Is hosted VoIP always cheaper?
Compare complete written proposals over the same term. The result depends on your equipment, support arrangement and required user features.
Related guides
- Hosted VoIP for business
- Business VoIP options
- 4G backup broadband
- Static IP addresses for business broadband
- Business phone and broadband bundles
Sources
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