Hmm, well me being a dad, have experienced this, but on the flip side. Mother has no house phone, only cell phone. In the past, I bought a cell phone for my 12 year old daughter, phone is restricted to call myself, mom, grandparents, emergency (911).
Cell phone was to be off during school hours so we never really had that problem, my daughter would turn it off during school, which is fine for all of us.
However, my ex-wife would take phone away as a way to punish my daughter at times (frequently and sometimes for weeks at a time). While doing so, the ex-wife doesn't answer my phone calls/return to messages to speak to my daughters. Or if she did, the response is "they're busy right now" or "they're outside playing right now" or "we're eating right now" or "I'll have them call you later" and I'd usually get no return call from them. Currently the phone is broke, so I have almost no contact with my daughters while they're at their moms.
I'd say, KIDS deserve the right to have access to both parents at all times, parents are divorced -- not the kids and parents, and just because the parents have different "custodial" time, that doesn't mean you stop being a parent and being available to your child. I get the whole thing when it's being abused, or in the instance of texting one parent to "pick up early" for no good reason, but really?
People need to put their pride aside, and do what's best FOR THE CHILD, that means, allow both parents to be parents. There are mothers out there with some serious pride issues (I'm sure fathers too) and believe that just because they're the PCP, that they get to call all the shots. I understand when one parent has some serious parenting inadequacies, but let's face, that goes on in marriages too.
Maybe this falls under some different email thread though, I don't know...