iMessage thinks SMS # is a landline?

So I have this person that I've been trying to contact for a while and this device isn't an Apple device. So it goes to text messaging instead of iMessaging, but the problem is, is that iMessage seems to think his number is a landline. And he gets the messages and has tried to reply but I don't receive his replies. Also, I've had to pay $.25 for every time I send a message and obviously that adds up. It seems to think it's a landline. I have no idea what to do in this situation. And it's not my provider, my friend uses AT&T and I use Sprint. She tried it on her phone and got the same result. She got a text telling her the number was a landline and she had to pay in order to text it too. Help anyone?