Try the product helpline for your model of BT phone (towards bottom of the page below)
https://shop.bt.com/help/returns-and-refunds/bt-products-technical-help,-user-guides-and-after-sales...
if your model is not listed, have a look in the back of the manual where the relevant contact number for your particular model is usually listed.
Alternatively, if it is new and still in warranty, return it and swap it for another model.
(Some Googling returns quite a few mentions on BT forums about this issue as well as other types of random sound issues on BT phones happening unexpectedly)