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From a foreign country, you will need to dial 00 and 91 to reach India and 80 for Bangalore. The city has basic landline operators as well as many mobile telephony providers. BSNL, TATA Indicom, Reliance Infocom and Airtel are the four landline operators in the city.

• BSNL: The largest and oldest telephone provider in the government. Numbers start with the prefix 2.
• TATA Indicom: A relatively new entrant whose numbers start with the prefix 6.
• Reliance Infocomm: The newest entrant with numbers starting with the prefix 3.
• Airtel: Bangalore's most popular telephone service. Numbers start with prefix 4.

The main GSM operators in the city are:

• Vodafone: Has a relatively large user base and network.
  
• Airtel: It is Bangalore's largest telecom company. Airtel is owned by Bharti Group. Airtel numbers start with '9845' or '9880'or '9945'or '9980' and are 10 digits long.
  
• Spice Communications: A smaller carrier serving Karnataka state.
• BSNL:  Provides GSM services across Karnataka and much of India.
• Aircel: Aircel is a rapidly growing provider that acquired GSM licenses in 2007 and expects to begin operations in Bangalore and Karnataka in 2009.

Source:
http://en.wikipedia.org/wiki/List_of_Bangalore_telecom_companies

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 Bangalore. 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 Bangalore.