In September 2004, the number of mobile phone connections crossed the number of fixed-line connections and presently dwarfs the wireline segment substantially. The mobile subscriber base has grown from 5 million subscribers in 2001 to over 1,179.32 million subscribers as of July 2018. India primarily follows the GSM mobile system, in the 900 MHz band. Recent operators also operate in the 1800 MHz band. The dominant players are Vi, Airtel, Jio, and BSNL/MTNL. International roaming agreements exist between most operators and many foreign carriers. The government allowed Mobile number portability (MNP) which enables mobile telephone users to retain their mobile telephone numbers when changing from one mobile network operator to another. In 2014, Trivandrum became the first city in India to cross the mobile penetration milestone of 100 mobile connections per 100 people. In 2015 three more cities from Kerala, Kollam, Kochi and Kottayam crossed the 100 mark. In 2017 many other major cities in the country like Chennai, Mysore, Mangalore, Bangalore, Hyderabad, etc. also crossed the milestone. Currently Trivandrum tops the Indian cities with a mobile penetration of 168.4 followed by Kollam 143.2 and Kochi 141.7.