Snom phones just accept by default calls which are send from the same IP as configured as Outbound proxy or Registrar in the identities. Calls and messages from other IPs are rejected with "403 Use Proxy". The Phone Log will also print an error similar to this when a SIP request from a not-trusted address:
- The only exception is if the received SIP method is an ACK for an existing call, in which case the phone does not reject it, but it still prints the error "SIP: reject Request, not trusted address" in the log.
- This mechanism should protect against spam via SIP or unauthorized calls. In some environments though, the calling proxy is not the same as the outbound-proxy used by the phone for outgoing calls, for this reason it is possible to change the phone configuration to accept methods from untrusted IPs.
- With a high chance the calls are received from a server different to the outbound-proxy.
- The setting filter_registrar has to be set to "off".