I use Freepbx (Distro, version 6.12.65-29) to register to all my SIP trunks and behind it I have a legacy PBX connecting to FreePBX using a PRI. The difficulty I face is that whenever I make a call to someone’s IVR using the legacy PBX, I cannot use the * or # keys as FreePBX absorbs them as feature tones and I’m not sure how to program it to simply pass the tones. I would prefer not to disable all of the feature codes if possible as I do also use the FreePBX system to make calls. Any thoughts would be appreciated. 
It’s only the “In-Call” feature codes you need to disable/change.
I just gave that a try; it didn’t seem to make any difference.