If you have Microsoft Office 365 E3 licences you can add voice functionality by buying each user a “Phone System” user licence, then if they are to make and receive external calls they will each need a “Calling Plan”.
However, Microsoft Calling Plans are expensive, instead you could add a SpliceCom SV1000 and use it as a gateway to gain access to much cheaper VoIP lines for a different SIP provider, Microsoft call this “Direct Routing”
For some people a softphone on their computer or an app on their smartphone with the ability to make and receive could be fine, but most desk bound phone users prefer a physical phone, typical comments when using a softphone or app are suggested are:
This means you have basic users who require an expensive phone and an expensive Calling Plan, the way round this is to use a SpliceCom SV1000 as a gateway / "customer-provided Session Border Controller (SBC)"
The Microsoft Teams Phone System and the SpliceCom SV1000 act as a single system, yet the SpliceCom SV1000 can connect to far cheaper VoIP lines and supports low cost display phones for basic users as well as a full set of complex telephony functions and reporting that is not available on the Microsoft Teams Phone System.
A Teams / SpliceCom hybrid solutions lets you mix Free Teams softphone / app users with easy to use SpliceCom receptionist / complex call routing / call recording / call reporting / Call Centre providing extra functionality and keeping your costs down.