Some of our SIP devices suddenly stopped being able to make calls. After some investigation using sngrep on the Asterisk server I noticed that the INVITE messages are rejected:
← 401 UNAUTHORIZED (containing the WWW-Authenticate header)
I would assume that after receiving the 401 the client would send another INVITE containing the proper authentication header but for some reason it doesn’t.
At first I thought this is a client issue but what’s puzzling me is that REGISTER works perfectly fine:
← 401 UNAUTHORIZED
Even more puzzling is that when I connect the client to a different network (behind a different FW, 4G network for example) OR set the client to use TCP instead of UDP everything works fine. Also, the whole setup used to work fine on UDP until a few weeks ago.
I suspect that the firewall (Meraki MX64) is at least part of the problem but I have no clue what to look at.
Any hint on where to look would be highly appreciated!