Can I use my softphone on more than one device?

The short answer is: YES

You should install the softphone on all devices where you plan to use it (your work computer, your laptop, etc.) using the same license key for each installation.

The limitation is that you can only use ONE of these installations (using the same license key) at any given time.  So if you're on the softphone on your computer, and you launch the app on your laptop and try to make a call, your active call on the computer will drop.