Failed to authenticate device (incoming calls)

hello All,

some time - but not always - incoming calls will not go though, and I get the following error on the log file:

[2014-09-30 17:56:18] NOTICE[1773][C-000000d5]
chan_sip.c: Failed to authenticate device "02498****"

The caller receives a voicemail message from my provider, that pops up when our FreePbx is “unreachable”.

On the other side is another Asterisk, but unfortunately, my provider won’t give me instructions on trunk setup, probably because they don’t like me using Asterisk myself and not purchasing value added services from them.

Googling for this error, it seems related to Trunk, Incoming settings.
Because the Incoming section of my trunk is blank, I wonder if I should fill it with some information in order to route the incoming calls correctly.

Any idea is very, very welcome.

Thanks for your time,