Utilize the input text as a context reference, choose the right relationship between Tata Teleservices and Mumbai from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

The regulator further said that only Bharti Airtel in Mumbai and Tata Teleservices in Delhi met the laid-down quality of service (QoS) benchmarks for call drop rate, which as per norms should be less than 2%.

Answer: headquarters_location