OK, I know nothing about Nigerian service providers, but their systems will record all calls made and so they should be able to explain why the credit has been deducted. In other words, they can tell you what calls they think were being charged, when they were made and who to, and I would tell them that if they can't explain why the credit was removed then it is presumably an error in their system and they should refund it. Don't just accept them saying "we're charging you, we don't know why but we're going to keep your money".
If they insist on being jerks and thieves, presumably there are other operators you can change to.