I lost my Samsung Galaxy S, which was signed into Google Play.
Now, Play Store has identified that someone has inserted another SIM card with a different service provider.
Is there anyway I can find out the phone number of the inserted SIM?
Sounds like a stolen phone (theft of mislaid property). Report it to the police. In the US, if you have the esn/meid they can compel the carrier to give them all the info about who's using the phone. I'm not sure how these things work in other countries.