The 5 SIP errors you will actually hit

The full SIP code reference has dozens of entries, but in real debugging you will meet the same five again and again. Learn these and most call failures stop being mysterious.

The five you will actually run into

The full SIP code reference covers everything from 100 to 608, but in practice you will meet the same handful of codes over and over. Knowing these five by sight will resolve most call problems before you even need to dig further.

401 and 407: authentication required

This is not really a failure, it is the network asking you to prove who you are. The first request is sent deliberately without credentials, the server replies with a challenge, and the device resends the same request with the correct answer attached. If you see this pair on every single call, something is misconfigured; if you see it once at registration, it is completely normal.

488 Not Acceptable Here

Both sides connected, but they could not agree on how to actually send the audio, almost always a codec mismatch buried in the SDP. Check what each side is offering before assuming anything else is wrong.

486 Busy Here and 480 Temporarily Unavailable

486 means the phone is actively rejecting the call, usually because it is already on another call. 480 means the network could not reach the device at all right now, powered off, out of range, or simply not registered. They look similar to a caller but mean genuinely different things to a debugger.

503 Service Unavailable

The server itself is the problem, overloaded, misconfigured, or temporarily down, rather than anything about the specific call. This one points you at your infrastructure, not at the person you were calling.