This was used to test the speed of subscirbers dials by field service technicians. It could be called in via a special phone number, or connected into a subscriber's line by someone at the Local Test Desk.

1927 article on the dial speed tester, pg. 427: http://www.americanradiohistory.com/A...
Photo of Rainier office, then & now: http://revertive-pulse.tumblr.com/pos...