How do you get this opportunity? I've never seen it in my game... 
This opportunity is a very odd one and has a very unique set of parameters for your sim to qualify for even getting that call. He or she needs to have a "long term" relationship with a deceased sim of good friends or better, and their gravestone must be available in town somewhere (either in your inventory or the mausoleum in the cemetery). The higher the better and the longer the better, but it seems that 3 sim weeks is approximately the minimum. I'm not sure how it keeps track, but apparently it does.
Once you've achieved this, the call can come anywhere between a couple of hours and never. I've had sims who's spouses through 3 life stages have died of old age and never got the call, and sims who's friends at work died (whom they didn't know as long) and got the call almost instantly. But, on the bright side: once you get the call you can bring back any ghost as a playable sim. It doesn't have to be the sim that helped trigger the opportunity.
