Mahanagar Telephone Nigam [MTNL] — I am complaining about refund for deposit

Address:

A/13 Mahamaya bldg Tambe Nagar SN road Mulund West, Mumbai, Mumbai City, Maharashtra, 400080

I had installed mtnl telephone. At that time rs 8000 deposit was taken and it was said that it will be given when we terminate the service. Now when we terminated the service they are telling that the amount of deposit is deducted from the bill amount partially. But when we enquire about it, people said that the amount deduct in the bill is the interest of deposit and you should get the deposit. Please help me get my refund of deposit which i had paid earlier to mtnl.
I hope that this query will be solved asap.
Thanking you