The trunk will not register
- Confirm the username or phone number is exactly as shown in the Sipflex portal.
- Confirm you are using the separate trunk password, not the portal login password.
- Set the registrar and outbound proxy to
sip.sipflex.co.uk. - Confirm transport and port agree: UDP/TCP on
5060or50600, or TLS on5061. - Check DNS resolution, system time, firewall egress and authentication responses in the SIP trace.
- If a router interferes with UDP SIP, disable SIP ALG. Testing TLS or alternative port
50600can help identify that problem.
No audio or one-way audio
- Confirm the SIP dialog completes before changing codec settings.
- Inspect the SDP addresses and RTP ports offered by both systems.
- Allow expected UDP media from
52.56.188.70/32. - Check NAT mapping, advertised public addresses and local RTP ranges.
- Disable SIP ALG and avoid unnecessary SIP/RTP port forwarding.
- Temporarily prefer G.711 A-law with a 20 ms packet time to reduce variables.
- If using SRTP, confirm encryption is enabled and compatible on both sides.
Keypad digits or IVR selections fail
Use RFC2833/RFC4733 telephone-event DTMF. Do not rely on in-band audio tones. Confirm the payload is negotiated in SDP and inspect whether telephone-event RTP packets leave the application and reach the remote side.
Only one call direction works
| Symptom | Checks |
|---|---|
| Outbound works; inbound fails | Registration contact, profile assignment, number format, inbound route and firewall state |
| Inbound works; outbound fails | Authentication, dialled-number format, caller ID authorisation, balance and destination controls |
| Calls connect then clear | SIP response code, session timers, ACK routing, media timeout and NAT state |
| Wrong inbound destination | Full called-number format and PBX route matching |
| Wrong outbound caller ID | Use a number authorised on the Sipflex account and inspect identity headers |
Silent or ghost calls
First check whether the call appears in Sipflex call records. If it does not, the traffic may be reaching the PBX directly from an internet scanner rather than through Sipflex.
- Remove unnecessary public SIP port-forwarding.
- Restrict inbound SIP to expected sources.
- Use an SBC or firewall policy appropriate to the installation.
- Do not create permissive anonymous inbound routes.
- Review PBX logs for unsolicited REGISTER or INVITE traffic.
Collect evidence before escalating
Provide the calling number, called number, direction, exact UK date and time, connection method, transport, SIP Call-ID and response code. Include a sanitised trace when available, but remove passwords and authentication material.