Make sure you try another known working telephone before you contact BT.
A way to confirm this issue , while next to the landline handset , call your landline from your mobile , if you can hear ‘ringtone’ on the the mobile phone handset , but the landline phone isn’t ‘ringing’ , answer the landline anyway , if the call connects and you can communicate normally between the two devices, then you have effectively proved the ‘exchange’ isn’t sending the ringing signal ( current ) to the landline when it should , this ( as stated ) is a pretty common issue , primarily because of the age of the exchange equipment, however , it’s not detectable with a ‘line test’ so often when reporting an issue , your provider test the line and say no issues ,( they are not engineers ) .
This fault is known as ‘bell not ringing’ , and as stated is normally rectified by moving your connection within the exchange onto a different piece of equipment, ‘BNR’ is something of a throwback to older generations of equipment when the telephone instrument actually had a ‘bell’ inside it .