I just called this number for the first time as I joined (AARP/United Healthcare) part G last month. The system failed to respond to any number key presses on my iPhone (this after the long "if you already are a member or are calling for someone who is a member or if it's Tuesday between 9:01 AM and 9:02 AM, etc." announcement). I ended up repeating- well yelling- the number instead multiple times after which the attendant wasn't sure it understood, of course. When I finally got a live person she claimed she hadn't ever heard anyone complain about this system before. Perhaps it is only competent on a landline. What an introduction. It immediately makes me want to switch to another provider next year...
This is a tech problems and it happens not just on iphones - could be your phone in certain instances. It also seems to happen sporadically - not the same number all the time either.
Some seem to think that if you use cellular service rather than wi-fi when making the call, it helps. I have found that to be the case with my iphone. For some reason the wi-fi isn't recognizing the tones the phone is giving off and thus does not relay it.
Maybe try switching to cellular mode if it happens again to see if that corrects the problem.
Some say that if you put the number into your contacts, that it helps - I don't know about that one.