DTMF recognition issue when calls come from at&t VoLTE

Hi -

I’ve got a really odd one… I have a very basic setup with FreePBX running on a raspberry pi and an incoming analog trunk (actually a cellular device) connected through a Grandstream HT503.

Everything works swimmingly, outbound calls are placed with no issue and incoming calls get routed appropriately. My issues started when I added an IVR to the mix (as a result of setting up extensions for two close neighbors who are sharing the line)

When I call from my cell phone (at&t) with VoLTE enabled - my dtmf tones are not recognized. When I turn VoLTE off and fall back on older voice tech, the dtmf recognition is perfect. I’ve duplicated the behavior from another at&t phone in my family. If I call from the landline at work it is also handled flawlessly.

I’ve called various IVR systems (bank, cable company, etc) and all work without an issue from my phone over VoLTE - so the issue only happens with calls to my FreePBX IVR.

Any ideas on anything I can look for? Since calls from landlines, Vz phones and with at&t over the older tech work well, I’m a little hesitant to try too much on my own.

Bonus: I’ve figured out a lot in my 2 years playing with FreePBX at home and now at our campsite (this system) but I’m still very much novice level.

I appreciate any input or insight anyone can provide.

Since you have a gateway it would be at the gateway the problem exist since it’s taking the dtmf from analog and converting to SIP. Focus on the gateway and what it offers for debugging.