To be honest, these are not friends. Friends seek you, ask you to do stuff together, share their stories, thoughts, fears, dreams with you. Want your opinion and advice on things going on in their lives. Call you to ask how it went when they know you had something important (good or bad) going on in your life. Call you to share important going-ons in their lives. Introduce you to their other friends and want to meet yours. Confide in you things they don't tell others or they tell very few others. Email you things they think may be of interest to you.
Even if they insist they never call because they're "busy", it's just an excuse. No one is ever too busy when they're truly interested in a certain friendship.
_________________
There are two means of refuge from the miseries of life: music and cats - Albert Schweitzer