I don’t know why it is so difficult to find contact information for business customers on T-Mobile website. Did you try it? For the last few weeks I am trying to switch some phones from my personal account to business account and I always get lost with their automated telephone system.
If you need to contact T-Mobile here is the fax or email for business customers:
Fax: 877-214-5937, businesscare@t-mobilesupport.com