you have a similar setup to me, but it maybe better to just get a few things cleared up first, how many B&O phones do you currently have ?
1) Business - you are getting this line ( PSTN ) or POTS line from your "Baby Bell" or CLEC, and plugged into the socket is a B&O phone ??
2) Home - as above, but maybe a different provider and different phone
3) VoIP - this will be a dedicated VoIP phone ( please let us have the make and model) or an ATA ( analogue terminal adapter ) which you plug an ordinary analogue phone into like Vonage provide ( and others )
4) Gates - I am assuming this is on an intercom system, or similar
what I did was buy an Atcom PBX, the IP02 with the FXO module will do nicely
http://www.atcomshop.ca/index.php/pbx/ip02.html
you then plug your RJ11 cable for your business line into the first FXO jack on the Atcom IP02 device, you plug the RJ11 cable for your home line into the second FXO jack, you buy a normal Cat5 ethernet RJ45 cable and plug that into the IP02 at one end, and into your 8 port computer lan switch at the other
you then plug into your switch your VoIP phone ( or ATA ) , whatever was provided to you, this should continue to work as before provided you have made the right cable connections, and your ADSL ( Broadband ) router is also connected into your switch.
you then buy an FXS adapter ( also called an ATA ) for each B&O phone, and connect those back to the switch using ethernet Cat5 cables
http://www.nch.com.au/kb/10050.html
then you need something like the Doorbell Fon from Home Depot for your gates
http://www.doorbellfon.com/products/28wt/28wt.htm
buy another FXS adapter for the Doorbell Fon, and also connect back to the switch
you should now have 2 x PSTN lines coming into the Atcom IP02 PBX, and 4 x voip devices ( phones ) going into your 8 port switch
you now program the Atcom to route all calls to both your B&O phones only