Double dialling woes

I’m using the following script to try to dial two people and bridge the calls:

echo channel: SIP/$1@voipdiscount > /tmp/$1
echo callerid: $2 >> /tmp/$1
echo context: doubledial >> /tmp/$1
echo extension: s >> /tmp/$1
chmod 775 /tmp/$1
mv /tmp/$1 /opt/local/var/spool/asterisk/outgoing/

Here is the relevent part of extensions.conf under the doubledial context:

exten => s,1,Dial(SIP/${CALLERIDNUM}@voipdiscount,30,r)

Seems simple enough, dial the first number, setting the callerID to the second number and configure the context to dial the number specified in the callerID. The log looks good, gets as far as “Attempting native bridge of blahblah and blahblah” but I get no audio.

I would appreciate it if one of knowledgable chaps could point me in the right direction.

edit: Forgot to mention that this is my second stab at the double dialling malarky. It does work properly when I dial the first number into a parking extension and then call the second number into 701 to pick up the parked call. This should mean that my audio problem isn’t a NAT issue.