Call flow:
CUCM(10.5.2)---------(wan)--sip trunk---------3925VG SIP gateway-----------E1---PSTN
when making external call to PSTN, user unable to hear ringback intermittently.
when user A is on call with PSTN external number 1 , and same time any external number 2 calls user A then number 2 does not hear ring back or VM and also does not display the on phone screen. (Intermittent 2 bad 2 good)
when user A is on call with PSTN external number 1 and same time Internal user B calls internal user A, then User B is able to hear ringback.
under sip trunk sip profile is configured with mandatory MTP and Rel1xx disabled.
On gateway rel1xx disabled as well
Appreciate help
