I’ve same problem after Virgin landline migration to broadband – can call out but no reception of incoming calls. It took 3 VM “engineer” visits before I was told there wasn’t a fix Have been using two 6000 remotes and a 2500 handset for decades. They’re design classics, quality products which last a lifetime, I love the aesthetics, and on a practical note I’ve not found a replacement for 6000s which wall mount.
BT equipment I’ve bought as replacement does receive calls, but would be surprised if it lasts a single decade