Thanks Pratik & Ulf for yours reply.
I am very much fine
as per my findings, both are right : if we have number listed in About Phone section of android then only we can get number other wise we get null.
The second thing i come across :- i checked one Android Mobile(@Pratik : its amitesh mobile) in that mobile owner changed its sim card but MSISDN number(mobile number) did not changed , it remain old sim card number. So in this scenario we can get mobile number of our own but old number.