How can I identify unknown callers effectively?

The number from one friend using an iPhone to call my landline shows two mobile numbers joined together . Theirs is the second ' part ' , the other number is unknown . I have actually called the first part of the number and someone answered ( I put the phone down immediately ) . The friend had no idea who's number it was when I showed the call log from my landline phone . Creepy or what ? I treated it with suspicion and have blocked the number - since exactly the same thing happened just a month ago . My own network provider had never heard of this happening . Friend recently moved from 02 to Lebara . The number showing was 22 digits long , beginning with 09573 . The zero in front of my friend's part of this number is always missing . Is this a network problem or iOS problem ?

[Re-Titled by Moderator]