My advice is... forget about him having his own phone. Make some time to talk to him on a one to one basis.
I hate the dynamics built around mobile phones and children, to explain my point I am going to describe how easy it is to create bad feeling for the confusions caused over a mobile phone.
At the beginning, my ex would ring DS in the land line at regular hours and everything was fine. Admitedly, DS was often not keen in spending more than a couple of minutes on the phone with his had (or anyone else!).
Ex started ringing DS at different hours and things started to go down the drain: if he rang to late demanding to talk to him, I was obviously not happy of waking up DS to talk to his dad when I have spent an hour trying to get him in bed. If he rang in the early afternoon, we were either not at home or there where other distractions around (other children visiting, tv program, play etc) which meant DS really didn't want to go in the phone at the precise minute his dad called. Dad blamed me for this saying that I was not allowing him to have long conversations with his son or that I was not picking the phone when he rang (notice I don't have any device to identify who is calling).
When I asked ex to ring at certains times to ensure DS would be more likely to be willing to spend more time on the phone, my ex thought that as me trying to be controlling. I highly resented that, as I was just trying to ensure they could be on the phone free of distractions.
So ex resorted to the damned pay as you go phone, and I say damned because it brought more problems than it solved:
First, I really had to had it hammered into DS that the phone was there to talk to his dad, not to waste the credit downloading stuff from internet or ringing me to turn his light off or bring him something to eat (yes... the little monster found that increadibly funny), I would have been happy to add credit to the phone, but his dad refuse totalk to me and therefore give me the details to add more credit to it.
Second, once the mobile phone was provided ex demanded daily calls from DS. Problem is, DS didn't see the need to call that often so, often forgot to ring, which meant the short time they spend on the phone was spent mostly on ex telling DS off for not calling him (for some strange reason my ex won't ring DS but demands DS to ring him). Things went from bad to worse, my ex started punishing DS harshly during contact times because DS had not rang him as often as he demanded. DS then got into a circle where if he forgot to call he was scared shitless to ring as he was afraid of his dad telling him off, and I was equally scared shitless of DS not calling as I knew that I would be also blamed for that. But at the same time I could see the distress of DS at being told off and punished so I feel that pressing on him ringing his dad was like taking him to his dad to get him a good smacking (sorry... it was that bad). At the time, DS case worker asked me to leave DS to choose when to do the calling, and as you may imagine that was the end of the calls.
Third. I can have a heart attack everytime the phone goes missing, simply... I can't replace it. He won't talk to me, he doesn't understand reasons, and if I get DS a new phone with a new number because he misplaced his dad's, I'm sure my ex is going to kick off.
Fourth. The Fibs... I don't blame DS for telling fibs to his dad (I'm scared too), he sometimes says he's misplaced the phone, sometimes he turns it off and he forgets about it without me even noticing, and he has even told his dad he didn't have the phone available during holidays even when I have carried the stupid phone over three continents to ensure the access was there without neither of them bothering to call each other.
So my advice is, go back to the basics if you can, reasure your child you are there for him. Take time off to meet with him if there are importatn things to discuss. Try to mend things with your ex so you don't have to walk over egshells all the time and get in knots about phone issues. And above all, please be re assured that most times, when a child doesn't call, it is because they are distracted having a good time :-)