Alright,

Even if you made all those international calls to Nepal in 'outside' of the promotional offer of free calls for Nepal - call T-mobile and explain them that either:
-You were unaware of the charges per minute for international calls or
-You thought T Mobile was still offering complimentary phone calls to Nepal during the time you made all those calls

Talk to them, explain them that you cannot afford to make that payment and that you have been a valued T mobile customer for a long time etc. Whatever good reasons you can provide. They will most likely reduce your charges to a minimal probably to a few hundred dollars. If the person you're talking to at T mobile s not able to give you a good deal, ask to speak to the supervisor but be nice to them. Polite but firm.

Hope it helps. Good Luck.