DTMF issue with IVR

When IVR set-up using GSM card, at times when the caller tries to dial an extension while still in the IVR only some digits are passed through resulting in an invalid extension. At times the dial to extension works. This is only on an incoming call on GSM card. If you dial the IVR extension from another extension, it works 100% of the time.
Anyone seen this and found a way around it?
Tim