Can't send iMessage from iPad to iPhone

[farzad.shafai]

I have an iPad with iOS5 and I've set my apple ID with iMessage (Messages).
I can easily send messages to other iOS devices set with Email addresses. But I can't send a message to people who use iPhone.
In fact, when i type someone's phone number (who i know uses iMessage with that phone number) or add their contact, after a little thinking, a red exclamation mark appears next to the number and it says "[number] is not register with iMessage". But if that same number texts me with iMessage, I can reply their message.
Can anyone please help me?! It's driving me crazy...!

 

[warghh]

when you input the person's mobile number, are you putting it in the field mobile or iPhone?

 

[normanbates]

It all depends on how they have their iMessage set up. If they have it deliver to their email address and haven't set the iMessage to their iPhones to 'deliver at two addresses' (one of them addresses being their phone number) you can only iMessage to their iPhone via their email address

As a test type one of your friends email addresses in to the address field and it should be fine