P.B.
Using the phone coupled with mail is great! Keep the phone short and sweet. Ask some probing questions - not just how are you. Things like "What did you do special today?" "What are the names of your friends" "What are your favorite colors"
Then mail - kids love to get mail. Send them a postcard from a trip or just from home. Write a note - and if they are old enough - print so they can read it. Send them little things - like $5 in McD's gift cards or some fruit snacks.
My sister (who is like a grandma) has been sending goodies like sidewalk chalk, used jammies from a co-worker and stuff like that. She does not get to spend as much time with my 2 and 4 year old as she did the others, so she is really trying to build a relationship. She old gets to see them once or twice a year. But when she does - she makes it count!
Kids just want time! Not money and material things. They just want you to make a fuss over them. She treats them so special when she is here. Just little things like playing with them in the bath, Playing a game with them, reading books - just watching them do things like ride a bike and telling them how cool they are!