[Serusers] session Timers

Hi all, I have a a doubt related with RFC 4028 - Session Timers in the Session Initiation Protocol (SIP). I am developing a new ser module that supports this RFC. While doing tests, I found the following situation: phone1(addpac) SER phone 2 (grandstream) Invite -> <-422 (Lower Min-SE) Ack -> Invite -> Invite -> <-481 (no such call) <-481 (no such call) What I saw was that, in the second Invite, the addpac sends the Invite with the the "to tag" that was attached by SER before (422 response). I belive that this is the reason of the 481 response from grandstream. I think it is a bug of Addpac. Is that correct? Thanks in advance!