UCM 6208 Simultaneous outgoing calls through GSM device which has Two SIM cards

Dear;

Can you please advise regarding below issue, 

UCM 6208 Simultaneous outgoing calls through GSM device which has Two SIM cards (GSM device is Matrix SETU VG DEVICE).

Now we can call outside extensions from one Phone , but calling using two phones is not possible, one phone is ringing and the other mentions that the services are used.

Checking with Matrix support, the system has capacity to receive 8 calls , i have added another SIP trunk but still same issue. Can you please advise.