Sounds like a problem with the phone's hardware. I'm curious whether the digits * and # are also affected, since they all use the same DTMF low tone component.
If the phone is under warranty, I would contact Adtran Support.
Is this only when interacting with an IVR or similar on a connected call, or does it also affect actually placing a SIP call? If it affects placing a SIP call, I concur that it's likely a hardware issue. If calls consistently complete to the correct number but only interacting with an IVR once the call is set up is broken, it could be something in the DTMF handling on the call path, such as RFC2833 vs. inband and/or the codec.