Yes. When you sign up one of the steps is to choose a phone number. This phone number will be connected to your account no matter how many polls you create. It will also remain if you renew your plan before expiration.
By default, the phone number will be removed from your account in 30 days after purchasing a plan or signing up for a free account.
Theoretically, a phone number setup for your account can always stay available for you if you constantly use the service.