What landline providers cannot contact you ?, if mobiles are OK , is it all of them , Vodafone, EE ,O2 being the major ones .
If your phone is now connected to the broadband router , then it’s not a line card problem , that would affect every type of call including calls from mobile networks , but DV doesn’t use ‘line cards’,
As already stated , it could be number portability if the number were previously a TT number , or a BT number that was ported in the past to TT from BT , or could be something to do with now being on DV.
You could try asking people who cannot get through calling your landline ( but presumably contact you using an alternative method ) confirming what networks are calling from, Sky and Talk Talk run their own networks , so not all ‘landline’ calls made to your number originate from BT’s network , this was true even before DV
