I used to be able to get my BT Yahoo calendar on my phone using Outlook.
I had to delete my email account from Outlook last week and when I tried to add it again, using the "Yahoo" option [instead of IMAP which doesn't support calendars I'm told], I now get a BT screen coming up stating that BT no longer support the Yahoo link.
So how do I get my BT Yahoo Calendar on my android phone now ?
I tried the Yahoo app but got the same BT message as above.
I tried a few other apps but all came back with the same BT message.
my email address is firstname.lastname@example.org so I reckon this is part of the reason the BT message keeps coming up everywhere.
The error you are seeing is BT have removed setting up email using the "Yahoo" option, you have to setup using "Other". This looks like support for the Yahoo mail app has been removed - they never supported this officially.
There is a workaround - normally calendar can be setup using CALDAV and ios devices support this natively but Android devices don't. You can use a 3rd party app on the Play store like DAVdroid
On the DAVdroid app - setup a CALDAV account using the server address https://caldav.calendar.yahoo.com and your email username and password.