Depends on your number formats.
In the UK, mobile numbers start 071, 072, 073, 074, 075, 07624, 077, 078, and 079, with "Personal numbering" on 070, and Pagers on 076 (except 07624).
Non-mobile number start 01, 02, 03, 08, 09, with 04 reserved, 06 possibly going to be used for "Personal numbers", and 05 a bit confused.
So you could write a regex for that - but it wouldn't work in France, Italy, the USA, Indian, China, or a huge variety of other places where the numbering conventions may be different.
So start by looking at the numbering format for the country you are interested in, then get a copy of
Expresso[
^] - it's free, and it examines and generates Regular expressions.