I have been to those that allowed nothing except for a 2-hour daily visit for approved visitors and I have also been on a lower security ward with cell phones allowed and internet in every room. Th e internet was filtered, whatever that means.
I don't think there is much benefit to anyone for a person who is acutely super-ill to have access to their mobile phone or the internet. I feel people in that situation need as little outside stimulation as possible--just my take.
It was nice having a phone when I was less sick and able to use it responsibly. I did a lot of texting with people and there was some support there. It does get a bit boring being on a ward day after day, month after month, and for me, year after year, so having the internet really was nice for me. I was able to email a few people and I think that helped me. But having that available when I was psychotic would have been awful. Firing off crazy emails, joining websites, etc. There is a time and a place for everything.