So, please can you help me understand this? I have a contract with you from 2 phones, 1 is pay as you go the amount of airtime I get on it is sort of okay

So, please can you help me understand this? I have a contract with you from 2 phones, 1 is pay as you go the amount of airtime I get on it is sort of okay. The other is still under contract and I must load additional airtime to make more calls. This morning I bought an R100 of airtime. I called 808, according to that I have more than R600 available, but when I try to make a call I get a message that I have no airtime available?