All landline numbers are treated the same now. This has been the case ever since landline porting was allowed.
There is a simple reason for this.
If I port my Virgin Landline number to another provider (eg. Plusnet) the incoming calls are still routed through Virgin even though I cease to be a Virgin customer. Similarly if I port a Plusnet number into Virgin, the incoming calls are routed via Plusnet. Virgins billing systems don’t know if a number is ported, just which provider OFCOM originally allocated the number too.
Cable customer since 1993. Services: FH TV, Sky Sports/Cinema (2xV6), Talk Anytime Phone, 350M BB, Virgin PAYG Mobile
The only way to pay nothing for geographic (01 and 02) and UK-wide geographic (03 and 030) calls is to subscribe to Talk Unlimited. I also have Talk Weekend which I sometimes find useful but I was once caught out by forgetting to put the phone down within 60 minutes and got charged for the excess.