Hello, I have Cisco 3640 with Voice and E1 card connected to Nortel PBX for call out from PBX to PSTN. I have problem when 1. user hang-up the phone and got dial tone from PBX 2. dial prefix 9 and telephone number (9,024455566) 3. user will got dial tone again from Cisco 4. If user dial telephone number again, it can go to PSTN. (024455566) User ------ > PBX ----- > Cisco 3640 ----- > SIP Server ----- > PSTN Have you ever found this case? I have tried to use command "isdn overlap-receiving" on serial interface. Thank you Ake