They're allowed phone calls, but not unmonitored ones. As in, there is someone listening to what they say on the phone.

That's part of the argument for why those phone calls are so expensive...

Monitoring what someone does on the web would be possible too, of course. At least if you only let them do it for a few minutes at a time like a phone call. Monitoring someone for an hour or more of web browsing is likely to be deathly boring for the one doing the monitoring.