my boys are with their dad one weeknight a week (pick up & drop off via school) plus one 24 hr period at the weekend (alternate Fri/Sat and Sat/Sun) so in the normal course of events I (RP) see them daily and don't feel any need for phone contact. I don't think they do either, they don't contact me and I'm not aware that they have wanted to and been denied that.
For their dad, he sees them for (part of) 4 days every week. He doesn't contact them by phone when they are with me either, and they rarely (less than a handful of times in nearly 5 years) ask to call him. Before we split he was away a lot and didn't have phone contact with them then so I guess it isn't out of the usual. They are aware that they can call him, but I think because they do seem him regularly and phoning hasn't been part of their lives they just don't think to do so.
If either of us are away with them for say a week or more we facilitate phone contact with the other parent.
I think if they had less physical contact, say every other weekend, then I would be more likely to encourage them into phone contact in between.
ds1 (10y) has recently got an ipod and will now occasionally email me (and his grandma - she loves this) and message his dad which is nice, but I think both his dad and I are being careful not to "intrude" on the time with they are with the other parent too much.
It does change with age, as the parent stops needing to facilitate it so much and the child takes over.
With a smaller child they are less likely to talk on the phone (what they are doing in the present is much more interesting) and it is probably best to have pre-arranged times where they won't be busy and so will be more likely to talk.
With an older child, the child can make the contact, and with technology it can be in a multitude of ways (my partner's 14yo messages him on Viber quite a bit).