Reliance Communications — non receipt of the rent

We had entered into an agreement with reliance infratel ltd in the year 2008 for the duration of 20 years for establishing a bts comprising of erecting a communication tower to contain antennae and installation of a pre-fabricated shelter to accommodate various electronics, battery bank, air conditioners etc..
We are not receiving the rent for the past 15 months ie.. From the month of june 2016.

When we approached the concerned department in this regard, we always get the negative response from the department or the manager stating that the payment needs to be processed from mumbai and they are helpless in this matter. And further for no reason they are refusing to provide the contact details of the mumbai office (Head office).

Please help us on to whom do we need to approach to address the issue.
This has been due from a very long time. Please help us on this.