Outbound DTMF issue

I am having a problem with DTMF working reliably outbound using Aastra SIP phones.

We have 6 Aastra 480i and 4 Aastra 480i CT phones. The RTP settings on all phones are:
RTP Port - 3000
Basic Codecs - unchecked
Force RFC2833 Out-ofBand DTMF - unchecked
Customized Codec Preference List - (empty)
DTMF method - RTP
Silence Suppression - unchecked

The Extensions are all set to dtmfmode=RFC2833 in freepbx

The trunk is vitelity. The SIP PEER info is:

context=from-trunk ; (this could be ext-did or from-pstn as well)

There is no firewall issue. The trixbox is on a COVAD T1 with a static, public IP.

On the 480i’s. if I set the RTP settings to RFC, voicemail and other system features that require DTMF are accessible, but outbound DTMF is still unreliable. Same issue with setting RTP on the Aastra’s to “Both”, except that the DTMF problems appear on different called numbers (for instance, Walgreens IVR now works but BofA doesn’t). If I set the RTP settings to SIP Info, outbound DTMF works reliably, but then I cannot access trixbox voicemail.

I do not have this problem with Grandstream phones with DTMF set to “in-audio” and RFC2833."

Any idea how to fix this problem? If I can’t fix it, I’m going to have to send all of the Aastra phones back and exchange for Gransdtreams or Cisco’s.

Try dtmfmode=auto in your sip.conf file for those phones. ULAW can handle inband but I’m not sure about GSM.