SIP/2.0 503 Service Unavailable
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Got SIP response 503 "Service Unavailable." back from xxx.xxx.xxx.xxx
Check codecs for your server and your provider. Often there are no matching and call is not possible.
503 "Service unavailable: no more gws" back from xxx.xxx.xxx.xxx - Provider does not have more open lines to serve your calls.
Usually this is provider problem.
Add international prefix to the number as described here
If device is unreachable, Asterisk returns such error to caller.
Check also:
