New user; I just got my 100.
I salvaged a phone from a friend's junquebox and got started. It took quite a while to grok how to load my Google account names and passwords into the 100 directly, vice surrendering them to Obi's app. But I finally did.
But I was plagued with hum imbalance. I tested the phone, something called a "Cobra" on a POTS line and had no problems. I was wondering if the Obi port was actually balanced. But to be sure, I found another phone, this time a real one that says "Bell System Property, Not For Sale" & I am happy to say, silence is golden. [I regard that generation of WECO gear as the finest phones ever made....] See this photo http://goo.gl/WJRS3 (http://goo.gl/WJRS3) of the setup; I bet I'm the sole Obi user with this configuration. It works quite well with GV.
There's a amusing sidenote: I called a friend whose POTS is habitually unbalanced. But he's on one coast, I'm on the other....and the power grids are NOT synchronized. You could hear the two hums drift in and out of phase s..l..o...w..y.
Close, Mine is a Stromberg-Carlson S-C 2500DPM (touch tone) Made in USA (circa 1968) Same as yours without the rotory dial.