iPhone DTMF digits in IVR duplicating

We are noticing intermittent issues where users are calling from iPhones on ATT network. They get to the IVR then the digits are not read properly. The user will type 4321 and 432211 or 44332211 is what Asterisk sees. The pattern hasn’t been consistent, and it doesn’t always happen. Anyone else experience this issue or have a solution?

This sounds like a misconfigured DTMF mode, or using an unreliable one. Check out voip-info.org/wiki/view/Asterisk+DTMF