asterisk users - Jun 2011 - Asterisk, attended transfers and DTMF mode

Hi,

Asterisk: 1.8.4.2

I've just managed to configure attended transfers using Asterisk and 
Grandstream GXP-2000 phones. The only way I've got it to work is by 
using one of the out-of-band DTMF modes on the phone (either RFC or 
SIP-info).

I think I can understand why - as Asterisk wouldn't be "seeing"
the DTMF
tones during the call if they are inband (or am I wrong)? I have 
"canreinvite=no" in sip config so that all media goes through
Asterisk.
However, it is strange that none of the wiki pages on voip-info.org or 
comments in the features.conf file seem to mention anything about this. 
Am I missing something? Can features/transfers/call parking work with 
inband DTMF? Is it an issue particular to Grandstream phones?

Thanks,

Sebastian