Not allowing your children to own a phone ≠ Force social isolation.
They swim, ski, kayak and play school volleyball and badminton so there's lots of opportunities for that sort of interacting but I think it would be pretty harsh to cut them off of their phones. If I could get 3/4 of their friends parents to agree I would for sure until they are 16.
and in many situation it is pretty much guaranteed to at least cause slight degree of social isolation, in some (like very rual) it might cause a large degree of social isolation
similar having a phone isn't a yes/no question, things like parent control exist, even through they often slightly suck