You can already do this! If you want to call a mobile phone or any number from Teams, just click the little phone labeled “Calls” then click the button that says “Dial a number.”
Note this is only available for certain licenses such as E3 with enterprise voice add-on and E5. You also need to have been given a calling plan by your administrator.
Please let me know if you have any questions.
Microsoft VOIP offering is confusing. Is it Teams, Skype, does it even exist? All the links divert to O365 with no mention of 'voice add-on' or calling plans.