KOLKATA: Public sector BSNL today said that it would invest around Rs 100 crore to upgrade the existing 2G/3G mobile network in the Calcutta circle.CGM of BSNL (Calcutta Telephones) S P Tripathi said that the network upgradation would be mainly to increase data speed and adding that the existing technology was having limited data capacity.Tripathi said that another 1,000 base transmitting stations ( BTS ) would be set up to improve signal availability across the Calcutta circle.With this addition, the total number of BTS would increase to 3042, he said.Mobile connectivity inside the metro railway tunnel was also being improved upon, Tripathi said.About landline, he said that the number of surrenders have come down. "The number of surrenders per month was around 2500", he said.On the other hand, the number of new applications were pouring in at an average 2,000 per month, he said.BSNL today launched a new value-added service by virtue of which customers could use their smartphones to make outgoing calls from their mobile, PC or laptop.