Having your PBX system on the cloud reduces your capex spending which typically will cost you anywhere from $2000 for a 25 user system right up to $25000 for a 300 user system. This is not including the handsets, the landlines and the labour needed to install as well. If you did choose the cloud solution, then the you would have to purchase the VoIP Phones anyway so that you can connect them to the PBX. Your opex would be 5% of the hardware cost to maintain and this is before you have even placed your first call.
There are many advantages for having your PBX serviced by an IP PBX provider in the cloud who would be able to get economies of scale on a multi-tenant system and therefore pass on the savings to you but the key factor in this solution lies with your ISP. There are many types of traffic that goes over the network into the internet. Common examples include http, ftp, video streaming, chat messaging and VoIP all of which is sent via data packets into the internet pipe along with your neighbour's data packets and into the general stream of data that your ISP manages. What if your neighbour has been hit by malware and all of a sudden, the amount of traffic going thru the bigger pipe gets bottled up? Or if they are streaming movies or downloading large amounts of data that have maxed out your ISP's internet capacity? Then time sensitive data packets like VoIP, gets delayed and hence this is how you get call dropouts along with other voice quality issues associated with your call. Some ISPs however do provide quality of service (QoS) that allows them to accept a prioritised data packet like Voice and ensure that this packet gets delivered first before any other traffic to the other end. Think of it as like a traffic cop on the road. In this scenario, your calls will never have voice quality issues. Assuming you are using something other than Skype which does not have QoS like say SIP then this traffic can be managed by your ISP. Most ISPs do not have this capability so the forum here is for people who have been searching for solutions and to discuss their ISP or knows of ISPs who do provide this service.