No header field via

I’ve posted this question in the users forum with no success.

From one moment to another i started having my cisco sip phone producing this message on the asterisk server:

Feb 3 03:35:55 NOTICE[22178] chan_sip.c: No header field ‘Via’ present to copy

The IAX phones continue working. The SIP phone cannot dial any number and it only produces that error on the asterisk server.
I guess i haven’t changed anything on the asterisk server or on the phone.

Other SIP phones (softphones work great). This must be a configuration change in the cisco 7940 phone.

What could be producing this error?

Thank you