MDM Locking device to ONLY Phone Functions?

This is more on the personal side, but it involves MDM, so I figure I'll try it here.

I have my son's iPhone supervised and enrolled in Meraki. Occasionally I want to be able to lock the phone down so it can only be used as a phone, so I set a profile to do Single App Mode with the app being "com.apple.mobilephone". On the surface this does just what I was hoping. When I invoke this profile, the phone app launches and nothing else will run. Home button doesn't do anything. I can switch between views in the phone app and do anything... except make a call. I can punch the buttons or contact, but when I hit Dial, nothing happens. Also, I can call this phone from another phone, it will ring and allow me to answer, but that's it. The microphone and speaker do not work. I should also mention that all other settings in this profile are enabled.

Anyone have any thoughts?

Thanks.