It's funny you should bring this up... I lost a friend's phone number just last week, and a different friend lost my number this week! We got all the numbers re-united again when we saw each other in person... but to answer your question, I think once or twice for a friend to lose your telephone number could be quite normal. More than 1-2 times, you have to wonder if the person is either deliberately trying to distance themselves, or are just totally disorganized
In any case if they are not making a priority of saving your number, you are under no obligation to 'fix' things or make extra effort... I would let the ball be in their court for now. Just my personal opinion.