Beocom 2, 4, 5 and 6000 are all DECT phones. I have no idea if you can connect them to a PABX. These phones have Caller ID.

Most PABX vendors will sell you an "analog adapter" so you can plug any good old POTS telephone into your PABX.

2-line Beocoms would presumably benefit from plugging into TWO "analag adapters" so you could benefit from the multiline features.

What about the Caller ID option? (If someone is calling within the PABX environment, does his number get displayed?)

Depends mainly on the analog adapter and the PABX itself.

I can tell you I have a little gateway (like a little-mini-micro PABX for a very specific purpose) and my Beocom 3 and Beocom 1 phones get caller IDs when they ring on calls coming from the gateway.  However, this happens only insofar as the gateway itself sends the correct signals.  

Bottom line, Beo will display Caller ID if whoever is sending the ringtone sends the correct signal with the correct timing in the correct protocol for the phone model.  Various Beocoms sold in various countries are generally sold to meet the protocol standard of the country of sale. 

Can I get the model number of your gateway? (basically, my system will used in France)

Sure, but like I said my gateway is for a very specific purpose.  When we are at home, our cellphones automagically link to the gateway and the gateway connects to all line-2 phones in the house.  So our mobiles ring on line-2 on the B&O house phones and when the B&O house phones make calls on line-2, the calls go out via the mobiles.

U.S.-standards.

http://www.myxlink.com/xlink_bttn.aspx