Black screen when call goes to voicemail

I've had this issue for the past few days:

If a call I am making goes to voicemail, when I take the phone away from my ear screen stays black and I cannot end the call. The home button doesn't work, lock doesn't work. I have to hold the lock and home buttons to restart the phone.

I've tried:

- wiping and restoring phone

- resetting network settings

- upgraded ios to 8.4

I can replicate the issue easily. Anyone else having this issue?