If you have plans to go abroad and want to temporarily suspend your T-mobile prepaid plan (not a prepaid card but a prepaid plan), you can suspend it (or hold it) for 30 days without loosing your balance. You must activate your phone with 30 days (or 29) to reclaim your balance and phone number though. If you don’t do it, your balance will be lost. Your phone number cannot be guaranteed as well after the 30 days.
Just call T-Mobile at 1-800-T-MOBILE (1-800-866-2453) and let them know you want to suspend your phone. You have to call within 29 and 27 days to reactivate your phone.
I got this information for myself from T-mobile customer service. Thought it might be helpful to others to save some money.