If you have 50/50 shared care do your DC ring/facetime the other parent on the days they are with you?
Ex wants DS to ring them and has sent a mobile phone. But I think it's confusing for DS and disruptive. I think ex wants this for their own aims and not thinking of the best interests of DS.
Recently divorced and living separately. DS is 5 and trying to settle him into his new routine in the best way possible for him.