The block was set up years ago, long before the current will was created IIRC. I think it's highly unlikely the parents had anything to do with it.
George is likewise very unlikely; again, unless we assume he's spent years plotting the perfect assassination for the Cambells only to ignore their children, there was no reason for him to worry about Jaye and the MC coordinating back then. We're also told he had to pull a lot of strings to get the MC back in time for the funeral. It's possible that was an elaborate lie, of course, but it seems an inordinate amount of work for minimal gain. It would have been much simpler and more effective to block Jaye on the MC's phone as well, which clearly did not happen.
Tara seems to think the MC was behind the block, and while far fetched, that makes a lot more sense. Our hero was, running away from his attraction to Jaye, after all. By blocking her ability to receive his calls ensured that he couldn't contact her and confess his feelings in a moment of weakness. If he ever really wanted to get in touch with her he could always come home (or arrange something via their parents).