How long is the service period on a prepaid starter SIM prior to adding minutes?

ewoo01

    I need to port my landline number to Google Voice, and half of the process is first porting it to a prepaid carrier. Already owning a still usable T-Mobile flip-phone, I thought I'd use T-Mobile. I know I need to buy a prepaid starter SIM package and activate it. How long of a service period do I have before the prepaid account terminates if I don't want to buy a $10 minutes card to top it off? I figure that I'd hopefully need a week at most to complete the port from landline to T-Mobile, then from T-Mobile to Google Voice.

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