Ground-start requires that both sides participate in a kind of "handshake". Think of it as a tiny, but extra step that your phone system has to make before a call can be placed or received. This handshake eliminates the possiblity of an inbound/outbound call collision or "glare".

Here's how it works: Your PBX looks for ground on tip; the CO looks for ground on ring. The absence of ground on the monitored lead assures each side that the other end is idle.

The PBX requests service from the CO by placing ground on the ring side. The CO detects this and responds by grounding the tip side. Your PBX sees this and says, "Oh good. He says that it's OK to place a call." The PBX then removes the ground on ring and closes the loop (user hears dialtone).

The near opposite happens during a CO-initiated seizure of the line. The difference is that the CO doesn't look for dialtone, but instead imposes ringing current across RING.

Taking a two wire butt set offhook by itself will NOT ground both tip and ring. However, unless you isolate the circuit from the PBX, you may find that you can draw dialtone by placing a ground across either tip OR ring (ground on tip looks like a CO initiated call; ground on ring looks like the PBX wants to place a call). My advice (and Jim's, I believe) then is to lift the bridge clips--or whatever--and take the PBX out of the picture before testing.

Bill simplified the whole thing by suggesting to first check polarity. If all is right with the world and this circuit, CO battery will appear on the ring lead (PBX side) when the circuit is idle.

"Press play and record at the same time" -- Tim Alberstein