Asterisk unable to register to external SIP provider

I've been trying to get my head around this.

Just noticed today that my SIP trunk isn't registreing at the provider.

The Asterisk log only shows timeouts, and I can't see anything in the logs on Sophos.

[2017-10-08 21:25:33] NOTICE[2462] chan_sip.c: -- Registration for 'xxxxxxxx@sip.viptel.dk' timed out, trying again (Attempt #4)
[2017-10-08 21:25:33] NOTICE[2462] chan_sip.c: -- Registration for 'yyyyyyyy@sip.viptel.dk' timed out, trying again (Attempt #4)
[2017-10-08 21:25:53] NOTICE[2462] chan_sip.c: -- Registration for 'xxxxxxxx@sip.viptel.dk' timed out, trying again (Attempt #5)
[2017-10-08 21:25:53] NOTICE[2462] chan_sip.c: -- Registration for 'yyyyyyyy@sip.viptel.dk' timed out, trying again (Attempt #5)

The SIP module is loaded on Sophos (tried unloading it which changed absolutely nothing)

Any ideas to test?