I suspect that, as someone else mentioned, you have registered your phone number for iMessage (if you previously had an iPhone). That's probably why some of your contacts trying to send you messages are sending them as iMessages (message app on their iPhones) which Android phones cannot receive.
In order to deregister your phone number from iMessage, go to Apple's support page:
https://selfsolve.apple.com/deregister-imessage/
They have instructions on how to do it if you still have your iPhone, and how to do it if you do not have access to your iPhone. Once that is done, your number will no longer be associated with iMessage which will make your friends' messages from their iPhones be sent as regular SMS which you will be able to receive on your phone.
If this is the case, that you were using an iPhone prior to this phone, I hope this helps.