SIP/2.0 503 Service Unavailable
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Got SIP response 503 "Service Unavailable." back from 23.45.67.89
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 22.33.44.55 - Provider does not have more open lines to serve your calls.
Usually this is provider problem.
If device is unreachable, Asterisk returns such error to caller.
Check also: