From a foreign country, you will need to dial 00 and 91 to reach India and 22 for Mumbai. The city has basic landline operators as well as many mobile telephony providers. MTNL and Tata Indicom are the two landline operators in the city. MTNL is the largest and oldest provider with over 2.5 million subscribers while Tata Indicom is a new entrant with about 200,000 subscribers.

The main GSM operators in the city are:

• Hutch: Formerly Orange, this is the largest GSM operator in Bombay. It has a large user base and  network. Hutch numbers start with '9820' or '9819' and are 10 digits long.
  
• Airtel: Airtel started operations in Mumbai in 2002-2003. It has rapidly grown in numbers since then. Airtel numbers start with '9892' or '9867' and are 10 digits long.
  
• Dolphin: Dolphin is the GSM branch of MTNL. It starts with '9869'. It is state owned and is the smallest player among the GSM companies.
  

Source:
http://www.nationmaster.com/encyclopedia/

Pre-paid cards

The prepaid SIM card with a world cell phone is a convenient, economical and efficient way to stay in touch while in Mumbai. All incoming calls while you travel in India are free. You only pay the local rate for local calls since the SIM card is prepaid, there is no hassle of a contract. You can get a pre-paid SIM card at any mobile shop in Mumbai.