Well, maybe. It still works I think and it is immediately available. With the BT service I think we have to log on to setup.
Okay, I'll try to keep it simple.
BT Essential Voicemail is free. It takes calls when you cannot. You dial 1571 to listen to your messages and you can even do that from your mobile when you are out and about.
BT Call Protect is free and comes as part of your Call Package. It helps to prevent unwanted nuisance calls allowing you to decide which calls you want to send to a junk voicemail.
I've absolutely no idea if Shell offer this or a similar service.
[Edit: I've just checked and Voicemail is an additional paid for service with Shell]