Choosing a VOIP service is not always down to finding the lowest price. Cheap is not the same as inexpensive. You'll need to think about what features you need to support your organisation both now and in the future.
Telephone services vendors are Grand Masters when it comes down to sneaking in additional costs/fees to the customer's bill. The headline price figure displayed prominently on a web site is rarely what you end up paying.
You'll need to consider what access you have to technical support for computers and networks. Once a VOIP service is working they generally work well. However there is often an initial need to tinker with network settings to get everything working properly. You might need to make some improvements to your data network to ensure good data security and a good quality voice on your phones. You should also have access to a technician during the operation of your service for when problems arise. Such problems are infrequent, but baffling for the lay-person. The level of data network support provided by the VOIP service vendor can vary considerably. Their support often stops at the point where the telecom data service enters your building.
If you are using VOIP phones they are generally just plug in and go provided they've been properly configured. Just as with your desk top PC the software hidden in the phones will need an occasional update for fixes, improvements and security patches. You may need technical support to ensure this update process takes place. Some VOIP vendors will do this remotely, others may give you no assistance on this matter.
You should also carefully check the contract period and how it is handled at renewal. You might pay monthly but still be enrolling in a three year contract with expensive exit charges should you need to change your mind. Make sure you have an exit should the vendor be unable to get your system working. Watch out for extra costs such as telephone number rental.
In assessing a contract make sure you know:
Contractually committed cost
"Out of plan" cost projections
When comparing phone call tariffs work out how many minutes you'll need per use per month then check what the costs will be if you exceed the plan. Some call plans tie the "minutes" to users with no transfer between user plans. Watch out for how call times are measured. Is it to the nearest minute, or are call times rounded up to the next whole minute. Call Set up fees can vary considerably between vendors; the financial impact depends on the call usage pattern of your business. You may find some vendors particularly BT round up the individual call costs to the next pence amount. If your business makes a lot of short outgoing calls the impact of call set up fees and cost rounding can be significant.
Despite all the considerations mentioned above you will almost inevitably save a lot of money on telecoms costs by moving to VOIP. You also gain considerably in business flexibility. However don't ditch your existing phone system until you've used the VOIP for a while and be sure to retain at least one incoming number on a traditional land line.