Hi,
it works for me because I only have to check if the Number starts with something like this:+4978455488 <== All numbers which are starting with +4978455488 are numbers of the location of my company. So if I dial a number which is starting with +4978455488 our Asterisk definitely can handle the call Internal.
So if I dial INTERNAL +4978455488389 I cut off the +4978455488 and dial only the 389 and ignore the +4978455488.