The answer isn't simple or straight forward - everyone has a different example or model - no two set ups appear to be the same - it's become more confusing than ever. The parts that definitely don't change are the network Infrastructure - you need a decent quality network switch - ideally managed, POE, and a we would recommend a gigabit switch, a newer router, and a reliable wiring infrastructure.
If you choose to get you phone lines over the Internet - then you’re really going to need to know about QoS - Quality of Service. QoS is very important - without QoS there is no priority of voice or video over data. This means that without QoS there is no guarantee of a reliable quality voice call.
Standard ADSL or Cable internet doesn't have QoS and it's not an option available from major providers. To get QoS you typically need fibre Internet - which isn't a guarantee of end to end quality but it is a much more reliable service than ADSL or Cable Internet.
QoS puts the lanes on the highway to segregate the traffic – without the lanes segregating the traffic you end up with Chaos. Fibres definitely a bigger & better internet pipe than ADSL / cable internet but without proper lane segregation with QoS enabled you can still end up with traffic chaos as different applications can interfere with each other and voice doesn't have a reserved amount of bandwidth to sustain a quality call.
A lot of people will recommend separate or dedicated internet connections for VoIP traffic and while that can help it's not a fool proof answer as the upstream providers don't put QoS on their own network and if the networks congested then you will still have voice quality issues. Also completely segregating the networks is not a fool proof design as unless the networks are properly segregated with vlans and proper routing statements then some functions may not work.
Think of integrated features like voicemail to email or unified communications - where you can dial from your computer screen - but if the networks don't allow the proper cross over data packet traffic then you wouldn't be able to take advantage of these features. This might be ok in certain limited installations - but quite often this design doesn't work out as the value of unified communications and redundant networks is usually more important and many features are limited by totally separate system designs – not to mention the double up in costs by requiring twice as much equipment when half as much would do.
NATG typically recommends a blended voice and data network with the proper QoS or voice segmentation aspects programmed. This allows you to minimize the double up on data infrastructure and take advantage of all the benefits of VOIP without any sacrifices.
Once the network is solid and in place you can decide if it makes more sense to host your own cloud or to purchase a piece of someone else’s cloud. Typically, owning your own hardware makes the most long term sense, however, no two applications are usually the same and it makes sense to consider various options and scenarios that match your business expectations.