First off, this won't work:
@Mob_num <> 'NULL'
That compares the number agains teh string literal "NULL", rather than checking for a NULL value:
@Mob_num IS NOT NULL
Should work better.
Secondly, this kind of validation is really something you should be doing in the presentation layer, where you can report a problem to the user and get him to fix it. Byu the time it reaches SQL, it's normally far too late!
Finally, your INSERT code doesn't refer to the @Mob_num variable, so it won;t get inserted anywhere, even if it does validate correctly!