I found this on wikipedia Looks like number start with 077 or 078 or 07x8 so if it's an 077 number you have your 11th digit. If it's an 078 or 07x8 number you have a fixed number of tries. You could also try calling one of the providers. They might help.... Mobile telephones 07xxx xxxxxx?mobile phones and WiFi numbers. Individual mobile phone companies are allocated different ranges within the 073xx, 074xx, 075xx, 07624, 077xx, 078xx and 079xx area codes. Changes to mobile phone numbers in the Big Number Change were mostly straight replacements, such as Vodafone customers on the 0378 block became 07778. 073xx xxxxxx Mobile phones (in use since November 2014) 074xx xxxxxx Mobile phones (in use since November 2009) 075xx xxxxxx Mobile phones (in use since May 2007) 07624 xxxxxx Mobile phones on the Isle of Man 077xx xxxxxx Mobile phones (former 03xx and 04xx?mostly Vodafone and O2 (formerly Cellnet)) 078xx xxxxxx Mobile phones (former 05xx, 06xx and 08xx?mostly Vodafone and O2 (formerly Cellnet)) 079xx xxxxxx Mobile phones (former 09xx?mostly Orange and T-Mobile (formerly one2one)) 07911 2xxxxx 07911 8xxxxx WiFi numbers (used by companies such as Tovo and Mobiboo)