1) If you come fron an iPhone, you had to disconnect your phone from iMessage while your iPhone was still on your account. If you didn't, all the iMessages they're sending you are still going to that phone (basically being thrown out). If you didn't do that, fo to a Genius Bar and they can set up a phone to do it for you.
2) Tell your iPhone friends to send you texts, not iMessages. Only iPhones can receive iMessages. Some carrier/phone combinations can convert them to texts with MMS, but it usually doesn't work (since the iMessage protovol is closed, so everyone trying to convert iMessages is really just guessing at what it is they're trying to convert).
3) If they have problems receiving your texts, each of them has to tell their phone, at least once, to receive from your number as text.
This is not a problem with your phone, it's a problem with Apple thinking that the whole world has to convert to their way of doing things - and it's not going to happen.