Where: A midsized apartment in the center of St. Petersburg.
What: The person who wrote this note is well informed about her neighbor's comings and goings; however, she makes it clear that this is not out of curiosity or a fondness for gossip, but is motivated by practical necessity. She is not worried about the fact that her neighbor has gone missing; she is concerned that the telephone may be disconnected.
Probably Tatyana makes more long-distance calls than anyone else and has for that reason assumed responsibility for paying the bills. Since she has not been home for several days, someone has to go to her workplace and find her. The phrase, "Please come by and see me," refers to the fact that the writer knows that the addressee takes classes right near where Tatyana works and she can try to find Tatyana there.