MTN charging for a service call

I had to correct my billing address on the MTN "My Contract self service portal" I repeatedly received an error code with an instruction to call 1761. I so did and could solve this issue as well as another on the contract. I found afterwards that 20 of my 100 inclusive minutes was deducted for the call. I find this unacceptable since I followed an instruction without any indication that this was a paid call.