Your phone has no way of telling whether a contact has an iPhone other than you putting the number in under "iPhone" rather than "Main" or "Mobile" etc.
Assuming you've done this, it might work until one of your contacts changes their phone and doesn't tell you, at which point it fails.
I guess Apple have decided it's up to you to keep track of who has an iPhone, Mac, iPad, iPod and therefore, who you can Facetime with.
Remember all of the above d evices can Facetime using an e-mail address
With my mums phone I've selected mobile in contacts after saving it but it has FaceTime button in her contact yet my friend has an iPhone and in contacts I've selected iPhone and save it and it shows FaceTime which is right. I have two people with an iPhone and the rest of my contacts are mobile and landline.