1. Remotely Hosted Telephony
IP Handsets connect through a company grade broadband connection (eg: ADSL, FTTC, FTTP, EFm etc) to a remote server (cloud based) which delivers all the features usually associated with a traditional telephone system. In most cases hosted telephony solutions can deliver intelligent phone call routing, such as simultaneous ring where an inbound telephone call can ring numerous locations– eg: IP phone, landline. mobile etc. In many cases a simple app such as Bria (http://www.counterpath.com/bria-iphone-edition.html) can easily enable your mobile phone as a softphone connected to your company VOIP account.

The key advantages of this kind of solution are Low Capex (no site based phone system needed) You can port your existing phone numbers to the Hosted VoIP service. You could take brand new numbers from any UK geographic area. Additional devices can be added as and when required. Hosted suppliers offer web browser interfaces or mobile apps for altering ring plans, call forwards etc

2. Site Based IP Telephone Systems (aka IP PBX, SIP PBX etc)
Mixture of Digital, Analogue or IP phones which can utilize Analogue, ISDN or IP Lines

The main advantage of IP Telephone Systems are usage of different sorts of handsets and lines. SIP Trunk or IP lines can be connected to enable non-regional telephones to be utilized. IP extensions can easily be connected at remote sites or used by remote workers who login when needed. These can easily be programmed like every other handset on the system and work as though they were actually in the workplace

Well there you have a brief summary on VOIP systems!