Numbers cannot be changed individually, their type is defined by the number block they are in.
In India ("IN") the numbers allocated to carriers for either fixed line, mobile or both.
If your number falls into a FIXED_LINE_OR_MOBILE block then it's possible that you could give up this number and the company could assign it to someone else as a new fixed line number.
You can examine the range data for India that the library uses here:
If some of these blocks are wrong (e.g. a block which says it is FIXED_LINE_OR_MOBILE is actually only for MOBILE numbers) then that can be fixed, but we get our data from official ITU and government documents, so we have no reason to think these allocations are incorrect.
HTH,
David