Asterisk - Nortel(Norstar MICS) Integration

Hello Everyone,

We are trying to integrate Asterisk / FreePBX with a Nortel Norstar system in the following way.

Asterisk=======PRI(Sangoma)=====PRI(Nortel)====Norstar=====CO 12 lines(Analogue)

Asterisk has the extension range 4xx and Norstar 2xx. We are able to dial both ways with no issues. The only problem now we have is When we dial out using the analogue lines using Norstar from an IP Phone in Asterisk, the line does nt get disconnected even after 10 minutes since we hangup the line on IP Phone. Any ideas / suggestions will be help full since we already spend 3 days on this.