Do you mean going via the one off payment on your computer https://my.bt.com/s/apps/appsbills/index.html#/logoutmakepaymentindex ?
Seems fine to me.
If you mean via the app there's definitely a problem https://community.bt.com/t5/Home-phone-including-Digital/Call-Protect-unavailable/m-p/2181761
I got a slightly new error message this morning (it's back to the same one above now):
I have been having the same problem the last few days, Service Unavailable, appears. That is using a Microsoft Edge server, if i switch to Google Chrome, everything works as normal.
Also works fine and able to pay bill on my Android tablet & mobile through the MyBT app, just have this "Service Unavailable" using Microsoft Edge on my laptop.
Microsoft Edge was updated on 2nd September. Might that be affecting those using that particular browser?