@Matt Cox 


You can check the rates here Microsoft Teams Voice Calling | Cloud Phone System


If you don't have the license it'll cost 2.5ppm for dialling a regular 0845 number, or 6.6ppm for a 0800 freephone number.


If you buy the license on top of E3 at £3.60 pupm then the regular numbers are included, its included as part of the E5 license..


It's hard to realistically compare costs to an existing service, your staff and a good proportion of your external guests will join using data, and so have no cost. I would also question the need for freephone in the UK, will your guests really care about a cost even if they don't have included minutes? The only place I see it being used is for some countries like UAE where you can't get national rate numbers.