No Ringback Tone in VoIP

1. Local vs remote ringback

Ringback can be generated in two places:

Local ringback (caller's phone generates the tone)

The caller's phone receives 180 Ringing without SDP or media. The phone plays a local ringback tone (a synthesized ring) until 200 OK arrives. This is the traditional behavior and works regardless of codec or media path.

Remote ringback (network generates the tone)

The caller receives 183 Session Progress with SDP, the media path is established, and ringback audio is sent over RTP from the network. This is called “in-band ringback” or “early media ringback”. It is used when the network needs to play customized announcements, regional ringback variants, or PSTN-bridged ringback.

Whether ringback works depends on the trace messages aligning with the phone's expectations. A 180 Ringing without SDP triggers local ringback; a 183 Session Progress with SDP triggers media-path ringback. Mismatches produce silence.

2. 180 Ringing vs 183 Session Progress

180 Ringing means the called endpoint is alerting the user. It is the canonical “phone is ringing on the other end” signal. Most phones use this to start local ringback.

183 Session Progress means the call setup is in progress and there is some early media to convey — typically ringback, but also announcements, busy tones, or Q.850 cause-derived audio for PSTN-bridged calls.

The key difference is what the caller's phone does:

The combination of 183 with SDP is the standard way to deliver remote ringback. The caller hears whatever the network chose to send — usually an actual ringback tone, but possibly a recorded announcement.

3. Early media and ringback delivery

Early media is RTP that flows before the call is answered (before 200 OK). It is what enables remote ringback, but it is also what most often breaks ringback.

For early media to work:

• The 183 Session Progress (or 180 Ringing) must contain SDP with valid media endpoint information
• The caller's phone must accept the SDP and open RTP early
• The network path between sender and receiver must be open before 200 OK
• The codec offered must be supported by the caller's phone

If the caller's phone does not support early media, it ignores the SDP in 183 and waits for 200 OK. During that time, no audio plays — not even local ringback, because 183 told the phone the network is generating ringback. The result is silence until answer.

This is why many SBCs convert 183 with SDP into 180 without SDP (or strip SDP from 183) when sending to legacy endpoints. The caller's phone then falls back to local ringback and the silence is avoided.

4. Common causes of silent ring

5. Codec mismatch during ringback

Codec mismatch during ringback is subtle because it can happen even when the answered call works fine. The 183's SDP and the 200 OK's SDP can offer different codecs. If they do, early media uses one codec and the answered call uses another.

If the caller supports the 200 OK's codec but not the 183's codec, ringback is silent but the call audio works after answer. The user reports “the call is fine but I can't hear the ring.”

The fix is to ensure the 183's SDP offers a codec the caller can decode. Most reliable is to constrain 183 to G.711 (PCMU/PCMA) regardless of what the answered call will negotiate — G.711 is universally supported and the temporary use during ringback has no quality impact.

6. Diagnosing no ringback from a trace

The diagnostic flow is straightforward once you know the four states:

• Was 180 Ringing or 183 Session Progress sent at all? If neither, the called endpoint is jumping straight to 200 OK. Configure it to send 180.
• Was SDP present in the provisional response? If no SDP and the response is 180, local ringback is expected — verify the phone is configured to play it. If SDP is present, early media is expected — check whether RTP actually flowed.
• Did RTP arrive between 183 and 200 OK? If 183 promised early media but no RTP arrived, the media path is broken upstream. Check NAT, SBC, and intermediate firewalls.
• What codec was in the 183 SDP? Verify the caller's phone supports it. If the 183 offered Opus or G.729 and the phone is G.711-only, decode fails silently.
• Did the call work after answer? If yes, the issue is isolated to ringback. The 200 OK negotiated a codec the phone supports; the 183 did not.
• Are there 100rel/PRACK requirements? Look for Require: 100rel in 183. If present and the caller's phone is not PRACK-capable, this is likely the cause.

Frequently asked questions

Why does my VoIP call go silent after dialing?

Silent ring happens when the caller's phone is waiting for early media that never arrives, when 180 Ringing is sent without SDP to a phone that expects early media, or when the codec offered in 183 Session Progress is not supported by the caller. The most common fix is having the SBC convert 183 with SDP to 180 without SDP for legacy endpoints, which falls back to local ringback generation.

What is the difference between 180 Ringing and 183 Session Progress?

180 Ringing means the called endpoint is alerting the user (the phone is physically ringing). 183 Session Progress means call setup is in progress and there may be early media to convey, such as ringback tones, announcements, or PSTN-derived audio. 180 without SDP triggers local ringback on the caller's phone; 183 with SDP triggers in-band ringback delivered over RTP.

Why does the call audio work but ringback is silent?

This is almost always a codec mismatch in the 183 Session Progress SDP. The 183 offers a codec the caller's phone cannot decode, but the 200 OK negotiates a codec it can decode. The fix is constraining the 183 SDP to widely supported codecs like G.711 (PCMU/PCMA) so ringback works for all callers regardless of what the answered call negotiates.