The subscriber base of other companies included in this data are Aircel, Uninor,Videocon and MTNL.
The subscriber base increased by 4.69 million or 0.66 percent compared with 711.55 million in April. This is the slowest growth registered in last six months.
Idea added 1.26 million new customers, highest among the seven telecom operator, while Airtel maintained leadership with maximum subscriber base of 229.44 million.
Videocon Telecom registered highest growth rate of 2.1 percent in subscriber base at 7.46 million in May compared with previous month.
In terms of net addition of new customers, Idea was followed by Airtel which added 1.19 million new customers.
Uninor maintained third spot in terms of new customer addition by adding 0.93 million.
Aircel pipped Vodafone this month by adding 0.7 million new subscribers, while Vodafone added only 0.43 million.
Videocon added 0.15 million and MTNL 0.01 million new customers in May.
UP East added 1.05 million, highest among all 22 telecom circles, and was the circle with maximum number of GSM subscribers.
COAI said the data does not include GSM subscriber number of Reliance Communications and Tata Teleservices. It has also stopped reporting the GSM numbers of state-run BSNL.