ISP/TELCO operator answers the call randomly

I got a T1 line with 23 DID numbers and using it with 2 TDM40B cards. I only use zap channels not DID or CID.

Everthing is working smoothly so far except one thing. Sometimes ,we have to call the same phone number 3 - 7 times. The phone numbers we call out are random numbers. It is either busy or ISP/TELCO operator answers the call. After 3 - 7 times, the call goes through. I entered ww in in front of extensions, but the problem is still there.

I have another * server with A@H 2.7 and 2TDM40B cards. The problem is the same.

Any ideas?