Q: iPhone switches to voicemail when I don't want it to

Do you mean that the phone is sending calls directly to voicemail instead of letting the phone ring a few times before sending calls to voicemail? If so, it sounds like Do Not Disturb is turned on. Go to Settings > Do Not Disturb, and set Manual to Off. If that's not the issue, this article may be helpful:

If you can't make or receive calls on your iPhone - Apple Support