[HELP]alcatel omnipcx+asterisk for conference call >3 usr

the small company where I work bought an alcatel omnipcx office r3.0 with alcatel dedicated phones (Reflexes 4020)

This pbx has been connected to 3 ISDN BRI lines for public calls and to an ADSL router for VoIP H323 calls among the 3 small branches of the company (which are equipped with an alcatel IP 4010 phone or an analog mediagateway)

One important feature that the company expected to find in the pbx is call conferencing with 5 or more simultaneous users (internal and/or external to the company).
Unfortunately the pbx can handle call conferencing with 3 simultaneus users only.

Can we implement this feature adding an asterisk pbx to the configuration?

I found on web this article:
and this fragment from the article gives me little hope :smile:

Connect to PBX via Voice over IP. If your PBX or local telephone provider supports SIP, H.323 or IAX voice over IP service, you can route calls to Asterisk via your LAN/WAN. This is the best option if it’s available as you can eliminate the need for T1 interface cards in the Asterisk box, as well as expansion cards for your PBX.

Could you please help me to study the right solution?

Thanks in advance!!