Mobile Phone Puzzle

[l5foye]

Recently I have had a number of missed calls on my mobile. When I phoned the numbers back, I get a message saying 'The number you have dialled is not in service' .
How do these callers use a mobile number which if you phone, you are told the no is not in service?

 

[Ted]

In the same way that you can withold your number when you dial,If you are a supplier or aggregator, you can brand an outbound call with any numer if you wish.

Often used by a supplier (say Vodafone) to send out multiple calls on behalf of a customer's call centre.