POTS Hybrid Does Not Hang Up Automatically
Scope
This covers all second-generation and newer POTS hybrids and some first-generation (ONE series) with Auto Answer upgrades installed. It includes the following systems:
- ONE and ONE+ONE with Auto Answer hardware installed
- TWO-x-12
- Nx12 / Nx6
- Hx6 / iQ6
- Hx1 / Hx2
The ONE-x-Six and older systems do not apply.
CPC Signaling
During an active POTS call, when the party on the far end hangs up, the central office will send a CPC (Calling Party Control) signal to the local device informing it that this has happened. A CPC signal is a momentary removal of loop current for around 400ms (this figure varies per CO equipment used). So the unloaded loop voltage on the POTS line will go from around 48V, to 0V, and back to 48V when the central office equipment sends CPC. If CPC is supported by the local device, it will automatically place the line back on-hook.
So why doesn't the hybrid hang up automatically?
If the central office or ATA (Analog Telephone Adapter) isn't sending standard CPC signaling in the form of momentary loop current removal when the party at the far end hangs up, then the local equipment will not hang up when the caller at the far end has disconnected.
The only solution to this problem is to contact the phone company or the ATA vendor and file a complaint that the phone line is not telling your equipment to hang up using this standard.
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