Well, the only way you can go underground is locally so...
Also, just talking to the same person 32 times in a row doesn't count unless someone leaves the communication first and going back into it. This can be done either through having either one person go back up or enter their secret base with the base closed.
For example: I talk to my 1 friend underground. I now have to talk to 31 more people to get Spiritomb. If I try to talk to him again when neither of us has disconnected, it is still 31. Now, I enter my secret base and close it. Then I come back out again. If I talk to my friend now, I only have to talk to 30 more people underground to get Spiritomb. I can repeat this as many times as I want.
Does this make sense?