Bot commands can only be used in the "ottspott-calls" channel once you have installed Ottspott.
Type ".call" and the phone number you wish to call with the country code first (no need to enter the "+" sign), then click Enter. Example: ".call 1650074110003" to call a number in SF (USA).
A clickable "Launch call in Ottspott" link is generated. Just click on it.
The call page will open in a Chrome browser tab and you can speak. If you never used Chrome to handle calls, you will have to allow the microphone first.
If you have multiple Slack teams (or using an Ottspott number on multiple teams), please make sure that you are logged in with the team you want to call before launching the call page.