I have introduced myself to "cousins" several times. I usually write them a letter first, giving them a definite "out" (by not responding) if they're wary or uninterested. So far, I've had nothing but positive responses. However, that's moot, as you apparently don't have the time for that.
If you're going to call them first (that's certainly the thing to do if you don't have time to write a letter), before you ask if you're speaking to so-and-so, introduce yourself, establishing that you're entirely willing to fully identify yourself, using your full name. I suggest you immediately establish your relationship to your g-g..-grandfather (not the people in the house) and you have no intention of entering the house. Present the most non-threatening person you can be.
Maybe, something like: "Hello. My name is Lianne Lavoie. I'm a genealogist and have been researching my great-grandfather [full name]. A land record indicates he lived in Fargo in 1880, at [give the address]. I found this phone number on the internet associated with that address [EDIT -establishes that you have nothing to hide about where you got the number]. I'm going to be in Fargo [such-and-such date] and, as I don't want to appear suspicious, would you mind if I took pictures of the house from the street or sidewalk?"
Thinking of it as if you were calling me, I probably would have no objection. After all, you could take pictures without my permission in any case. I'm guessing they would have no objection to that request.
Most of all, it lets them just say "yes" or "no" and hang up if they're uncomfortable or have no further interest.
Hopefully, the response may be "Really?! That's my great-great-grandfather!" and you're off to the races.