Short answer
A SIP trunk is an internet-protocol voice line that connects your phone system (PBX) to the public telephone network, in place of copper lines. FirstNet SIP Trunks add channels, numbers and a voice plan.
In detail: What is a SIP trunk and how does it work
Instead of one physical line per call, a SIP trunk carries many calls over a data connection. FirstNet SIP Trunks give you a set number of concurrent voice channels, phone numbers and a voice plan, delivered over FirstNet's ICASA-licensed (ECNS/ECS) network. The trunks work with any SIP-compliant PBX, on-premises or cloud-hosted.
What a FirstNet SIP trunk includes:
- Concurrent voice channels sized to your peak simultaneous calls.
- Phone numbers, either ported in from your current carrier or newly issued by FirstNet.
- A voice plan, either Voice Bundles with inclusive minutes or Voice Rate Options with per-minute rates, billed per second.
- A free backup SIP trunk with automatic failover if you also take FirstNet connectivity.
- SIP over TLS and SRTP encryption where your PBX supports it.
- 24x7 South African-based support and a published SLA on FirstNet-managed components.
How it works: your PBX sets up a SIP session with FirstNet's voice core, which runs on dual PortaOne clusters with zero-downtime updates. Calls then reach Telkom, Vodacom, MTN, Cell C and other operators through interconnect agreements, with multiple upstream interconnects for resilience and best-path routing.
What is free: the backup trunk. Customers who take FirstNet SIP Trunks together with FirstNet connectivity get it at no extra charge. It runs on a separate logical path, and if the primary trunk loses signalling, calls reroute to it automatically. Standalone SIP customers whose internet comes from another provider can add a backup trunk as a separate line item.
Sizing and compatibility: channels follow your peak concurrent calls, not headcount, so a business with 50 staff may need only a fraction of that. FirstNet plans bandwidth at about 100 Kbps per concurrent call plus 20% headroom. Most modern IP-PBXs work out of the box, including Yeastar, Cisco, Avaya IP Office, Mitel, FreePBX, 3CX, Asterisk and Panasonic, with G.711 as the baseline codec. Older PRI, BRI or analogue systems connect through a media gateway.
Commercially, channels are a monthly operating cost on 12, 24 or 36-month terms, calls are billed per second on your voice plan, and once-off items such as installation and number porting are quoted separately.
When comparing SIP trunk providers, check:
- Whether the provider holds its own ICASA ECNS and ECS licences, since only licensees can hold and terminate South African geographic numbers.
- How calls reach other networks, and whether failover to a backup trunk is automatic.
- Encryption, fraud controls such as destination blocking, and spending controls.
- Where support is based and what the SLA actually covers.
You can prove call quality before committing: a small FirstNet SIP trunk can run in parallel into your existing PBX, and the rest of the migration follows at your own pace.