I used to have Internet Call Display from Bell Canada, which is basically the same thing. (This is way back when.) I have DSL now, so it’s not an issue, but I got rid of the service a long time ago anyway, quite frankly because it was pissing off my friends.
Here’s why: I’d be on the web (surfing for porn or whatever), and a call would come through. I’d see who it was, then click the button that would give them a message basically saying that I knew who was calling, but would rather stay online, so please leave me a message. Rather rude, if you ask me.
Anyway, even though my service was from a different telco with different software, it worked very well. When I did choose to take a call, it would take a few seconds to log me off (meanwhile giving the caller a message to hold on), then my phone would ring. I do recall the first version being a bit buggy and unstable, but bear in mind, this is back in 1998.