Seurity risk by supplying ID no.

Cell C is billing me on the former contract rate instead of the new, lower rate. The branch at Woodlands insistes that I need to pay the old rate for 2 months after the new contract has kicked in. I complained online but Cell C refuses to help me unless I supply my ID number. In the light of the risk of identity *****, I refuse to supply my ID number to anyone over the telephone. I offered to supply the digits two by two while the operator provides the next 2 each time in order to verify who I am speaking to. She refused. Is there no other way of confirming a customer's identification? How do I get Cell C to help me with my problem? The branch at Woordlands cannot explain it to me either... HELP!