Paul,
Well modems seem to have no problems identifying it.
Most modems are even fine with the call progress tone if it ever does crop up (Rare unless you're calling a mobile).
Actually they're proposing to introduce a new tone to identify off network mobile calls to be inserted before the first ring.
It's become a problem since number portability came into effect. The mobile phone companies have taken to putting in announcements like "The person you are calling is no longer an O2 customer please wait while we connect you" when you call an 086 (o2's original prefix) number that's now on Vodafone or Meteor.
A lot of customers found the announcements repeditive, time consuming and annoying and others even hung up as it sounded like the customer had been cut off for non-payment or something.
They feel the need to tell you this as you might get charged higher rates if you call a number "off network".
