Contrary to popular belief, partition walls are not there to separate employees. They are installed to add dimensions to the already complex arena of inter-office communications. Don’t be afraid to “break the 4th wall”, pop your head up and communicate with your cube mates.
With this in mind, if your neighbour is having a particularly loud conversation, either on the phone, or with a cubicle visitor, it should be treated as a subtle invitation for you to show an interest and ultimately join in. Don’t let ignorance of the subject matter, or the fact that you have missed half the conversation, deter you. People are usually more than happy to fill in the gaps of any private conversation to someone who has joined late.
If your neighbour is, on the other hand, not particularly loud, then it is still an invitation, just a more subtle one.