[VoIP] Another WE question

[David Josephson]

Steph Kerman wrote:

>> <snip>
>>
>> A four-wire arranged phone still did all of its loop signalling on a 
>> standard loop circuit which carried the transmit (from the phone) 
>> path; this was connected to the network in the normal manner. The 
>> four-wire difference was that the receive path went directly (through 
>> only surge protection) to the receiver. I'm pretty sure the 3568 sets 
>> also had a small local side-tone generation circuit to provide a 
>> natural feel.
>>     
> The sidetone path was part of the line interface circuit in the Key 
> Service Unit, not part of the phone.  It also included a line amplifier 
> with low Z output to allow multiple telephones to be driven without 
> reduction of receive level.  However 3568 sets contain a crystal can 
> relay to transfer the receiver between the internal speech network and 
> receive pair depending on whether a 2W or 4W PU key is pressed.
>   
My 3568 has a plastic housing, "ice cube" type relay. There must have 
been a diode array in the KSU to determine the receive line to which the 
4-wire station should be connected. I have not seen such a 4-wire KSU.

There is certainly the capability in Asterisk to dump a call in process. 
Preempt tone is probably not in there, but if there were some reason ...

--
David