Telecom czar Sunil Mittal has a hectic schedule chock-a-block with meetings every day.

That is why T V Ramachandran, who heads the Cellular Operators' Association of India -- the apex body of GSM technology service providers -- sends him a missed call if he has something urgent to discuss, rather than disturbing him.

Mittal calls back when he is free. It's a code both understand well. Ironically, for Mittal's Bharti Airtel and the other eight major mobile operators in India, missed calls are becoming a growing headache.

Despite call rates being as low as Re 1 or less, ballpark industry figures show that over 30 per cent of all mobile calls are missed calls. They are deliberately made to convey a pre-agreed message or are calls that go unanswered.

A study by Learning Initiatives on Reforms for Network Economies (Lirne), a Denmark-based NGO that focusses on telecom issues, shows that over half of India's 140 million mobile subscribers make missed calls to convey a pre-agreed message.

As many as 95 per cent of the pre-paid customers used missed calls for this purpose, the study added.

For operators, missed calls clog networks without earning them revenue, also frustrating genuine callers with "network busy" messages.