[Freeswitch-users] DTMF and firewall

Hi, I'm using freeswitch to receive incoming calls from a sip provider namely AQL. When my freeswitch box is connected directly to the internet everything works fine. When I place a firewall/router inbetween the box and the internet, the software registers with the sip provider ok and answers calls but fails to respond to in call dtmf tones. AQL advised me to make sure I was using RFC2833 which I believe I have done by setting dtmf-type in my sip profile xml to 'RFC2833'. Can anyone advise me as to what other settings I should change to make the dtmf work correctly across the firewall/router? The router is currently set to allow all traffic. Many thanks for any help you can give. regards Andy