DTMF incorrectly recognized when IVR plays

I am using FreePBX with A2Billing, in a calling card scenario. When a user enters a destination phone number before the IVR message ends, the number dialed by the user is invariably recognized incorrectly.
For example, if the user dials 1905xxxxxxx the number is detected as 905xxxxxxx, so country code 90 is attempted. If the user dials after the IVR has stopped playing, the dialed number is detected correctly
Any help in trying to resolve this issue would be greatly appreciated.