I wouldn't...kids that young can't be expected to be super responsible with cellphones and neither your wife nor her ex husband can be trusted with them
maybe good old fashioned snail mail pre-addressed postcards with stamps and you make a point of sending one a week to them at their moms...get some made up that are pictures of you all having a good time or places you have been or places you want to go or things they like or funny animals etc...then they can decorate their rooms with them too
we did this with Cori's daughter when she moved back in with her mom...we made one of those corkboard things that are covered with fabric and then send her letters every week. We also bought her her own stationary (which she thought was pretty cool) and encouraged her to send us letters etc (all pre-stamped)
I also would not encourage your son to call his mom etc on nights he is with you...
I understand it is new but it also smacks of wallowing and the longer that sentiment lasts, the longer it feels that way
it is sad but there are also fun and good things and it is OK to focus on those too...
let her miss him those nights...she chose that
you are not the babysitter your mom is not her babysitter
