I feel ya, I am the exact same way. I doubt a lot of my friends because they aren't there when I need them. I only have 3 friends that I don't doubt. They are the only friends who I don't always think 'seriously, are you just being my friend so you look like you have more friends or something?'. In fact in the past I didn't even have that. I have started to distance myself from those people, if they don't have time for me I don't have time for them. If they can't be a friend, why be a friend? It is a bit difficult laying the line but I am just going to bring myself down continuously otherwise.
And the whole 'they aren't home' thing, I SO know what you mean, when you feel down and need someone, suddenly everyone has a life. You call them 10 times and they never answer, and when they finally call back you're feeling resentful and tell them to forget it, it doesn't matter.

If you want to know if they are really your friend, think about how much of a friend they actually are- do they stand you up, do they tell you they love you and want to do stuff and then you don't hear from them for weeks or months, do they suddenly disappear when you need help, do they lack empathy in your feelings (e.g. some of my 'friends' try and push me to drink, and they do drugs in front of me, even though they are aware of how uncomfortable it makes me feel).
Weigh up the pros and cons of their friendship, try and talk about it with them (without directing it AT them), look at how you feel about yourself, and try to figure out if some of your friends are really worthy. You deserve better than that.
A friend once said to me 'nobody has a right to bring you down, even if they don't intend to'. I think he was right. Some people are just so ignorant that they don't realise what a crap friend they really are, they don't see how their lack of concern and empathy is hurtful to you, and that it is going to reflect on how you feel about yourself.