I have a contract with them and when I need to call a landline number I receive a message that I have no airtime for the call

I have a contract with them and when I need to call a landline number I receive a message that I have no airtime for the call. So I reloaded my phone and right away I got a new message that I have R816 credit. I MUST CHANGE IT TO DATA BUT CALLING THE HELP CENTRE IS NO USE AT ALL! After I tried to tell them to change this how many times, nada! I have requested that a manager would return my call, but I am still waiting! I would not recommend them as your services provider at all!