It is a 'personal number' as it begins 070. These can cost up to 50p/min from a BT landline. Don't be suprised if mobile providers charge more than this. They do pay revenue, supposedly not directly to the receiving party, but in reality this still happens.
Mobile numbers begin 075, 077, 078 and 079.
There have been missed calls to mobile phones from 070 numbers. When you call back, you will be kept on the line and generate revenue for the scammer. It may be that the call has answered and you are listening to a recording of ringing tone, making you think the call is still ringing out. These numbers exist because people call them back!!!
This 070 number is charged at 50.10p connection plus 3.95p/min during the daytime from a BT landline. This appears to be the highest rate that they do.
Do you remember ringing back one of these numbers?