Typically, since a condo is owned by the individual owner, the phone NID isn't shared so there is only one for the condo unit, and usually on the outside but possibly in the basement/crawlspace or utility closet. In most cases, disconnecting at the NID is just a matter of opening the box and unplugging a short phone cord.
I agree -- that's the best way to do it. There are too many ways that trying to break up a "hope it's really wired that way" loop can go wrong.
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