New user with 1 post asking this? Assuming this is not edit spam or someone else is going to join and answer later.
That will depend upon many things.
What does it have to call (PCs you provide, random computers they might own, mobile phones you provide or whatever mix of android and IOS the employees might have)? How many employees are there (5 people is different to 50 which is different to 5000)? Does it have to integrate into your existing telecoms in some way? What sort of security or logging do you need? What kind of call volume do you expect? What is your budget (most "free" ones start to have things that make businesses prefer to pay once you get over about 5 people on the service)? Do you need group calling or is going to be one person calling another? Do you need screen sharing, presentation options, virtual whiteboard and the like? Do you need to send files or do you have something else for that? Alternatively is sending files around going to lead to bad habits if people are supposed to be storing files on your servers for auditing, recovery and whatever else? Are you going to run your own servers or do you want someone else to handle that?
For the boring and baseline there is a reason skype is as popular as it is. Works on most things (though might make phones hot), is pretty secure and has all the rest. Go above about 15 people though (and even that will get to be tricky) and you might start to come unstuck.
Many other things suck actually (I am still not remotely sure how that zoom thing got as popular as it did, and its security is abysmal) but they have their perks. Many will also want to tie you to a real phone number.
My work uses webEX. It’s more secure than Zoom, I think. Our schools use zoom, but for personal use, I’d use Facebook messenger chat thingy (which you probably don’t want to use for work). Zoom is nice, because you can share your screen. If you don’t need video, a conference call should be fine and free-ish.