You keep your credits after you cancel your subscription.
Why cancel if you still need credits?
Some customers only write minutes infrequently. For example, once per quarter.
Credits are added to your balance every month, and so this means you pay for credits you don't use.
To avoid overpaying, you can do this:
Buy a plan
When you don't need the plan anymore, cancel it
You keep your credits even after canceling
If you ever need more credits, you can re-subscribe to a plan.