More on IP Telephony
There are many different IP telephony service providers. Calls between telephones using the same server are free. Packages are available retail (Tesco, Skype, etc) and for business.
There are also PBX telephone switches that provide IP telephone extensions via RJ-45 extension sockets. Physically connecting these is easy, but configuration is difficult. If you want to go this way, consult an IP telephony specialist.
Power and IP Telephones
NOTE: IP telephones need mains power! If the mains fails, your IP telephones won't work, unless your entire LAN is protected by an Uninterruptible Power Supply (UPS). You need to think about this, in the event of an emergency.
IP telephones can either get their power from a transformer plugged into the wall, or over the structured cabling ("Power over Ethernet", PoE). PoE requires a special PoE Ethernet switch by your patch panel.