# Troubleshooting

Common SIP problems and how to fix them.

## SIP responses

| Code | Meaning | Check |
| --- | --- | --- |
| `401` / `407` | Authentication failed | Trunk username and password. |
| `403` | Not allowed | The trunk is enabled and your wallet has credit. |
| `429` | Caller ID refused | Use one of your Vonyx numbers as caller ID. |
| `404` | Number not found | The dialled number's format. |
| `480` / `486` | Callee unavailable or busy | Nothing on your side. Try later. |
| `503` | Service unavailable | You may be at your [channel limit](/docs/sip/limits). |

## Call connects but there's no audio

- Allow RTP on UDP `10000–20000` in your firewall.
- Offer PCMU or PCMA. Other codecs are not supported.
- If your server is behind NAT, advertise its public IP in SDP.

## Inbound calls don't arrive

- **Inbound calls** is on for the trunk, and the number is attached to it.
- The inbound address is a reachable host, without `sip:`.
- Your server accepts SIP from `43.204.9.41`.