Check voicemail capability when "1" is longpressed

TESTED:
- longpress "1", accessing voicemail
- call a prohibited number
- It was hard to reproduce the original issue, so I tried
  replacing "isVoicemailAvailable()" with "false" in the code and
  saw if it worked.

Bug: 5393977
Change-Id: I46eb3bcf425b34290fdebd8bea1c16bb37d10b8b