A parent can reasonably ensure that their child doesn't have a phone. Ensuring that they don't have access to a phone is much more difficult. I suppose that you can insist that your children only socialize with other children under your supervision, the effect of which is twofold: you can see that they don't use a phone, and they will be much less likely to have friends in the first place.

It's very hard to not give them internet access at all though even if you don't give them a phone, because they need it for homework and you can't and don't want to supervise them every moment they are doing homework.