For enterprise-wide Unified Communication you integrate your branches into your IP telephone network.
How to integrate a FreePBX telephone system into my IT environment? How to convert analogue or digital telephony (ISDN) to IP telephony? Here it is explained
The currently most versatile open source IP phone system presents itself and is rich in features, easy-to-use, stable, secure ... oh yes and flexible.
Models of the product families FreePBX and PBXact at a glance
Who decides on IP telephony software - must ask the question of whether open source or commercial software for him / her is the right one.