I think I would take the direct approach.
It would go something like this-
"Boyfriend/Girlfriend, I love the time we get to spend together on the phone. Let's do it more often OK?"
If they say OK- then they don't call, you may assume that there are reasons they don't.
They may have different interpretations of "more often".
You may be really needy and insecure and your definition of "often" would be in the category of "stalking".
Maybe you need to be the one to call.
Maybe they hate talking on the phone (my husband is like this) and it has nothing to do with your relationship.
Maybe it would be a good idea to figure out what "bothers" you about a lack of phone calls.
If they don't call, coupled with avoiding you in other ways- maybe it is time to seek a new relationship?