Preventing an iPhone to send automatically as SMS only when it's to a foreign number I have a friend who is currently in China. 

We talk a lot using iMessage. 
The problem is that sometimes the iMessage isn't delivered and my iPhone sends it automatically as an sms (= fee !!). 

I have enabled the option in the settings to do so, so that's the normal behaviour. But is it possible to enable it only for local numbers (not foreign) ? 

I pretty much doubt it, but there's no harm in asking the question :)
It's an option similar to one I've previously searched in vain for - short answer is there is no granular control over whether or not to send as SMS.

The safest option is likely to be to simply disable 'Send as SMS' altogether.
Settings > Messages