I see it is the Administrators who are quoting a geographic number for use from overseas and an 0800 phone number for use within the UK. So well done to them for doing the right thing, unlike other firms of administrators.
The other geographic number quoted on BBC Radio 5 Live of 02891 856547 for those customers in particular distress is actually the CAA helpline number.
Sadly the three other tour operators listed by XL on their website as being companies you could contact to find alternate return flights home with all use either 0870 or 0871 numbers and as we know 0871 numbers in particular are frequently impossible to contact from overseas while calling 0870 numbers is very, very expensive on many mobile phones from abroad. Now I appreciate this is not directly Excel's or the administrator's fault but as they clearly appreciate the issues with using NGN phone numbers from overseas perhaps they should have either visited this website to look for geographic alternatives or asked the tour operators in question if they would provide a geographic number, given the main place (i.e. overseas) that most of the calls are likely to be calling from in this case.
One does wonder why only three other tour operators are listed and there is no mention of BA, BMI or other scheduled airlines that customers might also organise other fights with. Not to mention national airline operators like Iberia, Olympic etc, etc, etc. One does of course understand why mentioning the phone numbers of Easyjet or Ryanair would be pointless but perhaps customers could at least have been guided to the URLs for their flight booking websites...........