Trying to understand your setup ....
a) You have a PBX with 4 RJ11 ports that are normally connected to analog PSTN lines
b) You plan to connect each OBi PHONE Port to one of the 4 PSTN Ports on the PBX
c) For incoming GV/SIP call on the OBi, it will ring the PHONE Port. The PBX then detects
the ring on its corresponding PSTN port the OBi is connected to, and invokes the
Auto Attendant of your PBX to direct the incoming call to another extension on
the system
More or less correct?
Some quick answers,
1. Yes. Easiest way to do this is to select "Trunk Group 1" as your PHONE Port's Primary Line.
By default trunk group 1 includes SP1 and SP2, where SP1 has higher priority. You can
change the order by modifying Trunk Group 1 config (or define your own Trunk Group; up to
4 can be defined per OBi. It's a bit more work but not too bad; refer to OBi admin guide for more info).
The Trunk Group concept is used for outgoing calls.
2. OBi itself can support 4 concurrent calls (your Internet uplink bandwidth usually the
limiting factor instead of the OBi; it may take about 100kbps per call).
However, since your method of connecting
the OBi to the PBX is via OBi's PHONE Port, there can be only 1 call per phone line.
So indeed you will need to have 4 OBi in order to provide 4 phone lines to your PBX.
This is the limitation of using analog PBX to interface with OBi.
(On the other hand, OBi can interface with IP PBX, such as Asterisk, using SIP; in that case
the number of calls is not limited by the number of PHONE Port)
3. Similarly, incoming call is subjected to the same physical interface limitation. You can only have
1 incoming call per OBi phone port. The 2nd incoming call, if accepted by the OBi, will be presented
to the same PHONE Port as call waiting.