Koodo Phone: Do I have to open a new account or can I use a phone card?

[Josslyn B]

I bought my sister's Koodo phone from her once she finished paying off the tab, so essentially the phone is mine. But would I have to start an account with Koodo or would the phone work if I was to buy generic phone cards? I emailed Koodo and they said that they don't have pay as you go plans and to start a new account. But if the phone is paid off then, shouldn't I be able to use a phone card? Pay-as-you-go is the more affordable option for me