Why did BT keep sending bills after father had died?

They charged him £70-£80 a month and my mother is £1,600 out of pocketMy father died in June last year and BT has continued to charge him £70-£80 a month. It has advised that he should have terminated the landline agreement, which is absurd – as he was dead. My 91-year-old mother is now out of pocket by over £1,600. Problems began when my parents moved in 2016. We phoned BT to transfer their phone and broadband to a new address and to inform them I would