I am not an expert by any stretch of the imagination, but I DO have opinion about child contact by absentee parents.
I encourage you to not be a Disney Dad or a Mickey Mouse Mom. If you are fortunate enough to get time to call your child and don't know what to say to them, tell them about your day. Ask their opinion about even something little as their views on broccoli. If they don't or can't speak coherently, keep the conversation going. They might just miss hearing your voice, no matter if you are telling them about changing a tire or reading your grocery list. Tell them something you like about them. They will remember. And be consistent. Call the same time each day, and find out what their likes (and dislikes) are. I think the time spent with them is better than any toy you might get them. Children grow, and there may be a day when you won't get this opportunity again. FIND THE TIME!