Can't text one certain phone number

My 3g iphone texts everyone fine except my friend's Motorola Razr. Any text gets kicked back with a "use a valid 10-digit mobile number."

I've tried texting entering his phone number as 7 digits (we have the same area code), as 10 digits (adding the area code) and as 11 digits (adding a 1 in front of the area code). None of these work. I have no trouble texting dozens of others within or outside my area code.

He gets texts from phones other than mine just fine.

Any idea what's up?

Thanks.
Carl