[asterisk-users] DTMF transmitting letter A

[Markus]

Dear list,

maybe not really an Asterisk question, but... all my users dial in via 
PSTN (via SIP DIDs) and enter a target number via DTMF through my 
Asterisk 1.4. Out of about 150,000 calls per month I see on average 
about 1 call per month where an arbitrary caller enters the letter 'A' 
via DTMF. These callers use their mobile phones to dial in. I just 
reread the Wikipedia article on DTMF but I don't understand how someone 
can send an 'A'. Any clue?

Thank you!
Markus