Digium D65 doesn't send DTMF when provisioned by EPM

[StefanoBrandimarte]

Hello,
we've a few Sangoma D65 Phones with expansions modules EXP-150.
If we configure the phone using its web interface, everything works well.
But, if we provision the phone through the Endpoint Manager Module, even if everything seems to be fine, the DTMF don't work anymore!
We tried with the very same extension, same settings, same everything. We tried using UDP, TCP and TLS. Everything works besides the DTMF!
If we perform a phone factory reset and configure again using its web interface, the DTMF work properly.
We're currenty using the free EPM module provided by FreePBX v17 (its version is 17.0.1.94).
Anyway we've the same issue also with FreePBX v16 (module version 16.0.86.30).
We also tried to update the phone firmware, that is now 2.9.27... and we tried also with the 2.9.25 firmware, provided by the EPM FreePBX Module.
Nothing seemed to work. We've tried more than one D65 Phone with same results.
There should be someting that the EPM performs, hidden from the web UI, that causes the DTMF interface to stop working.
BTW, we didn't touch the extension settings, so if the DTMF work when manually configured I reasonably expect them to work also when the phone has been provisioned.
Thank you.
Stefano