Hey there! Great question! Has your wife added you to her account as an authorized user? If so, you'll be able to access her account by giving your name and the PIN/passcode that she has set up. If you aren't named on the account, your options are limited. Paying a bill over the phone generates an $8 fee, and individuals who aren't specifically named aren't allowed to perform any action that will create a charge on the bill.
Your best option would be to log in to her my.t-mobile.com account and pay the bill from there.