VPN user stuck!

Hi everyone,

I use MikroTik 4.11 (RB750GL I guess) as a VPN server for my VOIP server. the problem is one of the user which is behind a Linksys WRT54GL got disconnected (phone GUI shows it is disconnected ) after a period of time while in the server it is still alive (no data is send or received). I did forward the proper ports on the server and I even did forward 1701 on the client phone.
The VPN passthrouth on the Linksys WRT54GL is active too.

Any suggestion for the reason or any solution for problem? for example checking the users in a period of time and re-connect them? Also in pc my user can log in with same user from multiple PC so why the phone has a problem with it?

Regards,
Saied