HI,
Actually they are two different apps and the approaches are slightly different.
The iOS version can check if the recipient is also iMessages compatible and will switch to SMS if Not.
This means there are extra steps before iMessages are sent to the iMessages server for delivery.
The Mac version can tell you the recipient is not using iMessages but does not switch to SMS meaning the action is Local to the device and not at Server level.
As far as I can tell the iMessages server works on storing full international Numbers as an ID which is then linked to an Apple ID.
Obviously the numbers can vary in the number of digits with some countries only having one leading + digit (i.e America is +1) and others have longer (i.e. the UK is +44 and Brazil is +55)
In the UK different carriers are separated in the next four digits a bit like an area code.
I must admit that sometimes I select an Name and the choice for an iPhone Number in the Contacts part of adding a name to the "To" field and it tells me it does not like the number.
Clicking and holding the red box at the pointed arrow end shows me the alternatives and I can uses the same number in an acceptable format.
Part of this is what is "registered" as the number with the iMessages server compared to other places
What people registered and what you have in your Contacts info may be the same but I would think the iMessages servers are expecting a minimum number of digits.
9:02 PM Saturday; January 5, 2013
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously