I have to disagree slightly with some posters here.
SOMETIMES, it IS ok to not answer the phone.
For example, whenever my ex calls to talk to the kids, it never fails that she calls at 5:30-6:00 in the evening. Right in the middle of dinner.
Dinner is very important to me. No, not the food. But my family still does dinner the traditional way. We all sit at the table, and each person tells a little bit about their day.
And I don't care if it's the Pope calling, the phone will not be answered during dinner. Period.
I have explained this to my ex umpteen million times. She just doesn't get it. I have told her the kids are home and available from 3:30-5:30 p.m. and from 7:00-9:00 p.m. every day. But of course she calls during dinner.
I always tell the kids that their mom called. They call her back, and 90% of the time, she doesn't answer.
Anyways, my point is that no parent should be at the complete mercy of the other parent. There are times when it's ok to just not answer the phone or turn the ringer off. When we have Family Friday Movie Night, we turn off the ringer. Not to block the ex, but because we don't want ANYONE interrupting.