Any experience with legal arbitration with T-Mobile over their not honoring written contract?

I contracted with T-Mobile for four lines with the Magenta Max program for a total of $150/month and they have verified the contract. They now say it was a mistake on their end and that it should have been a higher price. They have worked with me a few months but now say they will not come down to this price even though they have the contract and agree its there. Talking to supervisor did not help.

Does anyone have experience using the legal arbitration that we all "agree" to (no choice, of course) with T-Mobile? Any pointers?

Gracias.