There is a very simple answer to this question. We will get to that answer later on, but in the meantime we will show you why you need bandwidth when you add UCaaS to your system.
Even before you look at adding a cloud based solution to your small or medium business, first look around at the bandwidth that is being used by all of the desktop computers network faxes and other utilities inside of your business. Do you have any times of the day when it is more congested and things seem to move slow? Do your employees complain about long load times? Either of these is a symptom that you will probably want to upgrade your Internet service even before you take the step to unified communications.
Does your conference room host elaborate videoconferences? These can be real bandwidth hogs all on their own. If you host frequent video conferences with this type of equipment, you want to make sure your Internet traffic is robust anyway.
Quality of Service
When you look at adding a voice solution to your network one thing that comes to mind is quality of service. This refers to optimizing your network and prioritizing voice traffic over other types of traffic. By giving priority to voice even on a lower bandwidth system you don't have to worry about your phone calls sounding choppy and interrupted. Quality of service is set up in your local network.
Up and Down
Another consideration is unlike traditional Internet traffic that is mostly incoming with very little outgoing, phone traffic splits his time equally between incoming and outgoing. You may have fast Internet coming in, but if it is really slow going out it will not fit a UCaaS phone system properly. This usually manifests on satellite Internet systems, and sometimes rural wireless systems.
You should always run a speed tests at a reliable speed test website as you are looking to add any new service to your network. Note this speeds up as well as down, and look at Ping and jitter. Paying will tell you the amount of time it takes to run a full rotation between your network and another network on the Internet, which give you a real idea of how fast traffic is moving locally. Jitter is an indication of how varied that ping can be. High latency or high jitter can make your phone calls either fail or sound awful.
What Speed Should You Have
Okay here's the simple answer. For the voice of traffic side of that you're going to want about 100 kb per second up and down for every concurrent call. This means if you have 20 people on site and they spend about 50% of their time on calls you might be able to get by with one MB up-and-down for voice alone, but if they'll get all call of the same time it could get dicey.
This also doesn't take into account other network services. This 100 kb per second ballpark figure is in addition to your current network usage. Obviously, more is always better, but as you look to satisfy requirements without breaking the bank this is the number that you calculate.
Bandwidth is necessary for all cloud based services, and telephony is no exception. You can take steps using quality of service to prioritize your voice traffic but ultimately you'll need to have enough to run your voice as well as everything else in your business.