These operators, which have about 20 million mobile subscribers on their network in Delhi or 45 per cent of the total subscriber base here, have bought spectrum as their current licences are expiring by November-end.
"DoT should immediately call a meeting of both (Airtel, Vodafone) and arrive at a feasible solution so that consumers' inconvenience can be avoided," TRAI Chairman Rahul Khullar said in the letter.
Airtel and Vodafone have purchased fresh spectrum through auctions that were held in February 2014 as their permits are due to expire this November.
Mobile signals transmitted in 900 Mhz band cover almost double area compared to signals transmitted in 1800 Mhz band.
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"As reported by these telecom services providers, this whole exercise of change over of frequencies will need to be carried out in two stages...The above change over will be a huge challenge as all these changes are to be carried out in a live network...Any lapse may result in service interruption," Khullar said.
While DoT has issued Vodafone spectrum on October 10 after a delay of 8 months, it is yet to assign all frequencies to Airtel.
"The Authority is unable to understand the reasons for this inordinate delay in the assignment of spectrum despite the clear provision," Khullar said.
The Telecom Regulatory Authority of India had earlier recommended that operators be assigned spectrum 18 months before expiry of their licences so that they can make necessary changes in their network without compromising on quality of service.
Telecom operators paid for spectrum in the first week of March. The DoT has started allocating spectrum to companies this month but is yet to completely assign all frequencies, specially in Delhi to mobile operators.
"The authority is seriously concerned that this delay on the part of WPC (Wireless Planning and Coordination) in assigning spectrum in the 1800 Mhz band may lead to a partial breakdown of services offered by these two (Airtel, Vodafone) especially in Delhi, the national capital," Khullar said.