CDR Disposition on outbound calls

I have a customer who make the majority of their outbound calls over an IAX2 trunk, and the other over a PSTN trunk. Regardless of whether the call is completed or not (i.e. whether the other end answers or the caller hangs up whilst still hearing a ringing tone), the call disposition in the CDR is shown as ‘Answered’.

Is this a bug, or a limitation of Asterisk/FreePBX?

Asterisk version is FreePBX is 2.5.1. Call logs are held in MySQL.

Thanks in advance