The reason why you are unable to receive text messages from your friends who have iPhones is because you once had an iPhone that was attached to iMessage.
This issue has to do with Apple's implementation of the iMessage protocol. It binds device's and/or SIM card's unique ID to their servers (using the UUID from the device and the SIM ID) and doesn't release them easily.
So what happens is that Apple iMessage is taking over the text messages on your friend's iPhones and since you don't have an iPhone anymore, you can't receive iMessages anymore.
There is a potential fix for all of this...
- Go to: https://supportprofile.apple.com/MySupportProfile.do
- Log in if not already logged in.
- Click on ">" to the right of the phone listing
- Click "unregister"
If that doesn't work, get your SIM card swapped.