As well as 0123456789* and #, DTMF also has A, B, C and D.
Assuming the phone hasn’t sent it, the A could be generated when the GSM to PCM/analogue conversion is performed by the mobile network, or wherever the DTMF decoding is done inbound to your system. If you use a SIP provider, this could be as far out as them - you need to provide more information to narrow this down.
If you have SIP delivery of the calls, it would be unusual for Asterisk to do DTMF conversion, and trying to force it might well lead to unreliable decoding.