Call Hold, Resume, and Re-INVITE

Modifying a Call in Progress

A SIP call isn't static. After the initial INVITE/200/ACK establishes the session, either party may need to change it: put the call on hold, change codecs, add video, or update media addresses. The re-INVITE is SIP's mechanism for modifying an active session.

A re-INVITE is simply an INVITE sent within an existing dialog. It carries a new SDP offer that proposes changes to the session. The other party responds with a new SDP answer, and the session is updated.

Call Hold

How Hold Works

When Alice puts Bob on hold, her phone sends a re-INVITE with modified SDP:

Alice                    Telnyx B2BUA                    Bob
  |------- re-INVITE ------>|                            |
  | (a=sendonly)             |------- re-INVITE --------->|
  |                          | (a=sendonly)               |
  |                          |<------ 200 OK -------------|
  |<------ 200 OK ----------|  (a=recvonly)               |
  |------- ACK ------------->|------- ACK --------------->|
  |                          |                            |
  |  (Hold music plays)      |======= RTP (music) ======>|

The SDP direction attributes signal the hold state:

Attribute	Meaning
`a=sendrecv`	Normal bidirectional media (default)
`a=sendonly`	I will send but not receive — hold (I'm sending hold music)
`a=recvonly`	I will receive but not send — response to `sendonly`
`a=inactive`	No media in either direction — silent hold

sendonly vs. inactive

The choice between sendonly and inactive depends on whether the holding party plays hold music:

sendonly: "I'll send you hold music, but I'm not listening to you" — the common case
inactive: "Complete silence in both directions" — used when the PBX plays hold music locally or not at all

💡 NOC Tip: a customer reports "no hold music," check the SDP direction attribute. If the re-INVITE uses a=inactive, no media flows in either direction — the PBX might be playing hold music locally to the holder but not sending anything to the held party. If a=sendonly but still no music, the RTP stream exists but might not contain actual music (check the PBX hold music configuration). slug: "call-hold"

Call Resume

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