Help, deploying VoIP with Asterisk

Hello friends I need an advice. I want to implement VoIP in an enterprise. which is another building with offices distant from headquarters but are on the same intranet through MetroEthernet 2MB. at headquarters using a Siemens Hicom 300 PBX, in the other building not used PBX have 4 phones in total with several lines. as I can do to install voip?. I have to put 2 plants asterisk in each place is in the home and in the other building. or only at headquarters and in the other building install IP phones and connect with switch and since it has MetroEthernet would be on the same intranet. or is there any easier way to do it?

I think you could install 2 Asterisk PBX, because you would like to use the analog lines of one of the buildings using a Digium Analog Card, and at the other building replace the Siemens Hicom 300 PBX with the second Asterisk. Then connect both Asterisk with SIP TRUNK or IAX2 TRUNK.