Network topology: SNOM Phone ---- U1911 ---- eSpace 7910
SNOM phones cannot make calls to U1911. Once receiving the INVITE message, U1911 will reply "400 Bad Request" reject this call. As shown in below snapshot:
(P.S. All the captures are got in Lab, no information from real customer is included in this cases)
1. Check the "400 Bad Request" messange, we can see it marks the reason as Warning: 399 192.168.10.77 "SS210100F158L2673 Invalid refresher parameter in Session Expires header" as shown in below snapshot，so we are sure this problem is caused by something abnormal about Session-Expires header in the INVITE message.
2. Check the INVITE message of SNOM phones, (as below snapshot), we can see that "require: timer" header and "Session-Expires" header present in that INVITE message but "supported: timer" is missing.
3. According chapter 7.1 in RFC 4028, we know that all message from UAC must who supports RFC4028 must include supported: timer header, even if require: timer presents.
The INVITE message sent by SNOM phones not meet the description in RFC4028, supported: timer is missing
Advised customer reports to SNOM support to correct the INVITE message.