Microsoft Teams as a Phone System
If you have Microsoft Office 365 E3 licences you can add voice functionality by buying each user a “Phone System” user licence, then if those people are to make and receive external calls they will each need a “Calling Plan”.
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 from different SIP providers, Microsoft call this “Direct Routing”
Team Phone System only provides a basic level of functionality and uses expensive special Teams phones but the softphone and smartphone apps are free

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:
How do I answer the phone if there is a problem with my PC or it is off?
I don’t want to sit with headset on all day just so I can hear my phone ring!
Why should I use my mobile for work calls?
This means you have basic users who each 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 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 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.