Refer: https://support.t-mobile.com/docs/DOC-7261
How to add for a Pay As You Go (Prepaid) plan
To purchase a pass on your Pay As You Go plan, call T-Mobile customer service by dialing 611 from your T-Mobile phone, or call 1-877-746-0909. You can get a 1-day pass or a 1-week (7-day) pass. Automated account help and customer service representatives are available 24 hours a day, 7 days a week. For the hearing impaired, TTY service is available by calling 1-877-296-1018, 7 days a week, from 3 a.m.-10 p.m. PT.