if the keypad is appearing and the DTMF tone is being generated but is not registering with the calling toll-call system, you’ll want to check with your carrier. This might be with your carrier, or it might be a malfunction within the calling toll-call system.
I’ve never used this system, but this (option 2) would test what your iPhone is doing: https://testcall.com/804-222-1111/ — your carrier almost certainly has a similar testing system available within their network, though they may not publicize it.