Microsoft Teams as a VoIP Phone System
If you have Microsoft Office 365 E3 licences you can add voice functionality by buying each user a Teams “Phone System” user licence, then if those people are to make and receive external calls they will each need a Teams “Calling Plan”.
Microsoft Teams 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”
A Teams 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 call can 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, and the SpliceCom SV1000 can connect to far cheaper VoIP lines and supports
low cost display phones as well as a full set of complex telephony functions and reporting that is not available on Teams Phone System, everyone benifits. 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.
The obvious saving is that the SpliceCom SV1000 supports standard low-cost Yealink phones rather than expensive dedicated Teams only phones, but the biggest saving comes from not needing a Teams "Calling Plan" per user. The SV1000 uses the traditional telephone trick of sharing the lines among all users of the system, a typical organisation with 40 phones would have about 5 lines or channels in use at any time connecting them to the rest of the world.
Why pay for 40 channels (Calling Plans), when at most you use just 5?
Currently Microsoft are bundling the Teams "Telephony User" and Teams "Calling Plan" together, and giving a first-year discount, but the ongoing cost is £12 per user per month or £6 per user per month for the Teams "Telephony User" element, charity pricing is £8.40 per user per month for the bundle or £2.40 per user per month for just the Teams "Telephony User" element, either way you save £6 per user per month by bypassing Microsoft's "Calling Plan".

That £6 per user saving for an organisation of 40 people becomes £240 per month add the £25 per month link fee, then for existing SpliceCom Systems of this size the saving is £115 per month or £25,800 over the next 10 years, when using the Teams interface. Our customers normally keep their phone system for more than 10 years, and we are still supporting systems we installed in the 1990's. So, 10 years is a good compromise date to use in Total Cost of Ownership calculations.
If you don't have a SpliceCom SV1000, we can provide the integration via the £1.70 SV1000 virtual user licences, a £25 SBC link licence and adding some external phone "lines". Five VoIP channels with Free UK landline / mobile calls will add £30 to the monthly bill, together that is £123, resulting in a £117 monthly saving. The saving is even more if you buy rather than rent the licences, AND either way you get the fully power / flexibility / reporting of the award winning SpliceCom SV1000 included for nothing.
Truly spend less and get more.