Sending hookflash over RTP?

I am using an analog gateway (AudioCodes) and need to pass DTMF digit 16 to it to do a hookflash transfer. With Asterisk 11, if I send digit 16 from my client, it does not seem to get to the gateway. Is there some configuration I am doing? I’ve got it working by modifying both chan_sip and res_rtp_asterisk, but as this is an AsteriskNow install, I fear losing those changes and was hoping for a cleaner method.

If it works, it will be digit “f”. However there are some gaps in the implementation and without studying the code, I can’t remember where they are.