Why can't my friend text me on my iPad mini 2 from her phone which is not and iPhone? I cannot receive nor send texts to my friend on her phone, which is not an iPhone. The error message is "This phone number is not registered with iMessage".
iPad can only send and receive iMessages. If your friend don't have any device from Apple (iPod, iPad, Mac, iPhone), he can't send you a message through iMessage. Use another chat service.
(Like Messenger from Facebook or WhatsApp)