This is a problem another user on The Sims forums brought up, and I have verified in my own game. We both have playable ghosts that have friendships with other playable Sims in different households. The 'friends' are unable to call the ghosts on the telephone or invite them to social functions. It seems all phone interactions are disabled when the recipient is a ghost.
(The ghost herself can use all phone functions just fine. The issue is just that other Sims cannot call her.)
I don't know how this would work with 'non-playable' ghosts, as they're not really available all the time, yet you can form friendships with them, even before they become playable. I expect their 'dead' status is what introduces this issue. I would definitely like to be able to at least use the phone to contact playable ghosts. (If this could be extended to the NPC ghosts as well, with us just getting a message that they're 'not available' when they are not out actively haunting, it would be nice, but that's a bit less needed. Full interactions with playable ghosts are more of a must, IMO.)