Having had to have spent way to much time searching around to find out how to pay off the remaining balance on my installment plan here is what worked for me as of 2/7/2018.
- Dial *3 on the phone.
- Select "pay by phone" (not go to the app).
- Press 1 if calling from that phone.
- Press option 3 for the installment plan menu. (This menu only exists if you have an active installment plan on the phone otherwise option 3 is to hear your account balance again.)
- follow prompts to make payment to the installment plan.
Thank you for sharing @Pruch, I'm going to feature this post on the front page of of Community so it can help other people accomplish the same task.
In that case, let's help you out. Please check your inbox and reply to the private message I just sent you.