Voicemail DTMF issue

I have a Asterisknow 1.71 server updated to 1.8, generally everything is working ok.
One issue though is from a Grandstream GXP1200 which is offsite, if I dial into voicemail it rarely recognizes any keys pressed. Asterisk keeps asking for my extension# over and over, even though I do type it in. Occasionally I can get it to recognize my extension if I time it just right, but it’s quite rare.
Audio quality sounds great, latency <50ms. I am using g729 (purchased) and have also tried g722 and pcm to see if it fixed it, but same issue.
The grandstream is set to use payload 101 for dtmf, there is no setting to choose RFC or incall.

I have a softphone at the same site as the grandstream, and that works fine. it’s connecting to the same server, going through same firewalls, etc.

I can’t get to the site itself for a few days to try plugging the phone in locally to see if it works there.
Any thoughts?