I have a customer that reported a problem with their iPhone 6 and a 6 Plus not passing DTMF digits properly into various PBXs / conference bridges – most were using SIP trunks that I was able to test with. PRIs and analog lines all worked as expected. PBX logs all showed no digits being received as well. Android phones were working flawlessly on all the same tests #androidmasterrace
The newer iPhone’s (or maybe just a function of the newer IOS?) are now capable of some sort of Voice over LTE, I didn’t see any indication on the phone to show whether the call went over LTE or Cellular but once we changed the LTE settings to be Data Only, all the problem iPhones worked perfectly.
To disable Voice over LTE:
Go to: Settings->Cellular->Enable LTE->
Change the setting from Voice and Data to Data Only
Reboot your phone.
blogotubes 
Good morning,
apple could’nt help me – until I found this in the internet. BINGO! – that’s the solution!
Many thanks and greetings from Germany!
My iPhone 7 does not have the Settings tree structure described here. Settings -> Cellular ->
Cellular Data -> On/Off only, Cellular Data Options -> (Enable LTE -> On/Off only, Data Roaming -> On/Off only,
Low Data Mode On/Off only). So no way to set LTE to phone+data or data only.
With most phone systems on the other end, it works fine; with one system, it works fine most of the time, but about 10 percent of the time, it not only does not PROCESS tones, it does not SOUND them; a human on a phone in this system could not HEAR any tones!