RE: [Serusers] Session-Expires Header

Yes, you do need UA supporting this header. According to section 8 of the draft, draft-ietf-sip-session-timer-14: Session timers are mostly of interest to call stateful proxy servers (that is, servers that maintain the state of calls and dialogs established through them). However, a stateful proxy server (that is, a server which is aware of transaction state, but does not retain call or dialog state) MAY also follow the rules described here. Stateless proxies MUST NOT attempt to request session timers. Proxies that ask for session timers SHOULD record-route, since they won't receive refreshes if they don't. Here, SER is not a call stateful proxy but transaction stateful proxy. It will not maintain the state of the call. So, even if you insert the header, it will not change anything from SER's perspective. However, if as least one of the UA support this header, you may see a BYE request, which is good for accounting. If neither UA support the header, no re-INVITE or UPDATE will be generated and that doesn't help you more than what you have at the moment. So, until SER becomes call stateful (which is similar to B2BUA, but not the same), your best hope will be UA that support this header.