it's not uncommon for people with mental illness to avoid contact, not answer phone calls, not answer the door (if you can get there in person)
This. My DB is bi-polar, and has a very long cycle, so he is depressed for years at a time. He lives almost 200 miles away and is a recluse. Our only contact is when I text him every few weeks.
He was getting fortnightly visits from a CPN, because he had to have depo injections as he won't take his anti-psychotics, but he's only on anti-depressants now, so doesn't even have that. If he doesn't reply to texts, I ring his local MH unit to see if he's been admitted. If he's sectioned, they write to (mental health act requirement as I am his next of kin) but if he's admitted voluntarily, they don't let me know.
I could foresee something like this happening to him.