Strange SIP message

Hi all,

Since yesterday we get a strange message from some phones (at the moment 3 phones out of 20).
Also 1 user reported an issue when taking back a customer from hold, the line gets disconnected. This only happens on his phone and not on others but the issue got reported at the same time.

The server never displayed these errors before and nothing has been changed in the last days. The server is located in a datacenter and the phones here in the office.
Can someone explain these messages and help me to fix them?

These are the messages:

– Incoming call: Got SIP response 404 “Not Found” back from 172.36.13.208:5060
– Incoming call: Got SIP response 404 “Not Found” back from 172.36.13.69:5060

We use the freepbx distro (freepbx 2.9.0.7 and asterisk 1.8).
The phones are Linksys SPA942 and Cisco 504g.