All current mobile phone number series in India are rapidly being used up and coming closer to capacity, as the country's subscriber base is estimated to shoot past one billion by 2013.

In a report by the Times of India Monday, Rajan Mathews, director general of Cellular Operators Association of India (COAI), said there could be a "serious problem" if a new series of numbers is not brought in by the middle of next year.

"We are theoretically reaching the limit of existing number sets with a subscriber base of one billion," he said.

One solution is introducing 11-digit phone numbers--adding one more digit--although the Department of Telecom (DoT) is also looking at alternatives, he added.

Matthews explained the allocation of numbers to operators was done in batches, which considered factors such as the size of their subscriber base and efficiency in utilizing existing number sets. This batch system is used so as to avoid the chaos from having all kinds of phone numbers proliferate in the market.

An operator is allocated a particular range of numbers, and there are generally identifiers such as the first two digits, for example 98 or 99, that come to be associated with that operator.

However, none of the number ranges can be used to their full extent because of the way they are allocated, said Sandip Biswas, director at consultancy firm Deloitte, in the report. The percentage of numbering system utilization is around 50 percent, meaning only half or more of the potential numbers are used as phone numbers.

Biswas said phone numbers would have to increase by at least one digit to accommodate more devices and users, but "to ensure there's absolutely no scope for any problem, we may even see the introduction of 12-digit numbers".

Matthews said the Telecom Regulatory Authority of India (TRAI) had given similar kind of proposal to DoT. It was not taken up, although DoT did not reject the idea completely and is looking at alternative options, he added.

According to him, DoT said it may need remapping of networks and there might also be issues conforming to international number standards.

TRAI in the meantime has come up with other recommendations to help the situation for another three to six months, such as removing inactive users to free up phone numbers, Matthews noted.