Just ignore the phone calls and your contract will go over to month to month and you will be charged the same amount, just remember Telkom is always months behind so in most cases they will most likely just keep it on the contract it's currently on and keep charging your per month, even if you cancel your contract they will still charge you for months to come because Telkom can't cancel contracts it takes 7-8 people to finally sign of on a cancelled contract, which might also take months to accomplish.
Don't renew your contract because we will be seeing some changes in data pricing in 2016, even vodacom is slowly starting to drop pricing on their 10gb+10gb package, with a much better LTE coverage area than Telkom.