Outbound DTMF not working sometimes

Some services don’t work with DTMF control. For example, I can call in to my cell phone VM and have no issues getting or deleting messages. Tech support IVRs work fine for most companies. But UPS shipping tracking and our conference system don’t work for some reason (the latter one did provide some hints as it will occasionally recognize some of the numbers keyed in for the conference code, but when it doesn’t work it repeats the code back and its obvious some of the numbers are not being recognized).

Asterisk 1.8.11
FreePBX 2.10
CentOS 5.6

DTMF is set to rfc2833, as requested by our SIP provider.
Polycom Soundpoint IP 335 and 560 phones are set to rfc2833 for DTMF, as are the settings shown in FreePBX.

Is the signal/tone not loud enough, or not apparent? Not sure where to even begin looking since its obvious it is working in some places but not others.

One thing that can cause unreliable DTMF is if the tone is too short. Coding to RFC 2833 and reconstituting does change the length of the tone, although, in the case you describe, I don’t think the phone ever actually produces a tone (except to the handset).

Is there a way to set that to be longer? Or is it the SIP provider who is handling that part?