I like to have a list of questions put together before the exciting Christmas Day phone call. If I don't think about it beforehand, I inevitably forget to ask my missionary a question. Also, my younger children sometimes are shy on the phone and don't know what to ask beyond, "Hi, how are you?" I give them some ideas beforehand, and they pick what questions they want to ask. I have found our phone time be more productive and less time is wasted.
Also, don't be surprised if you are calm and collected before the phone call, but when the phone rings and you hear their voice, and start to talk, you suddenly become emotional. The first time that happened to me I was surprised because I was okay one minute, and not ok the next minute. Now I know what to expect. :-)
And don't forget to let your missionary talk to you about how things are going for him/her. Let him ask you some questions...or just let them have time to talk about what's on their mind.
Here are a few questions on my list. Feel free to send in other ideas to the group:
How many members in your ward/branch?
What is the church building like?
What is the town like? Rural, small town, small city, med. city, large city?
What does the area look like? flat? rolling hills? A lot of trees?
How hot is it? How cold is it? How often does it rain?
How is your companion? How long has he been out? Is his family LDS? Do
they support him with letters? Life member or convert?
Are you sleeping well? How is the food? What is the best food you've
eaten? What is the weirdest food you've eaten? How many dinner appts do
you get? What do you usually eat for breakfast? Lunch?
How are your clothes? Shoes? Socks?
How are your feet? Any problems? Do you need more toothpaste? Deodorant? Vitamins?
What is your apt like? Is it in a big building? Are your neighbors friendly or scared of you?
How far away is the Mission Office? How many in your District? Where are they from? Who is the District Leader? Zone Leader? How often do you have Zone Conference?
What do you usually do on P-day? What service do you do? How often?
How many investigators?
I'm sure you can come up with more personal questions, but this list will help you get started with some ideas on what to ask.
Merry Christmas! Enjoy those calls with your missionaries, and take notes on what they say. It is fun to go back a few years later and remember the phone call conversation.