Presumably they are more interested in information like which room someone is in than their position on Earth. I doubt that indoors the location accuracy of phone GPS would be accurate enough to be useful, particularly in buildings where many locations are designed to be secure both from physical attacks and espionage (probably a lot of thick metal plates in random walls). And even without interference issues, GPS elevation accuracy is pretty bad. Finally, you're only tracking the location of the person's phone, which in an emergency could easily get left behind somewhere.

None of this is to say an email chain is the optimal solution though.

"You guys all have phones right?"

The gov could write an app, installed on every senator's phone, that tracks their location as needed.

Perhaps the senators might be against such invasive surveillance? Then I'd hope their voting record on such bills matches their privacy concerns when they're the ones affected.