Setting softphone up with VoIP.ms SIP credentials
Click the headset icon in the top left corner, then Settings, choose SIP Accounts in the left window and click Add account button.
Now type your VoIP.ms credentials as it is shown on the screenshot:
The full list of SIP servers you can find on the VoIP.ms website.
Besides, for setting the softphone up you can also use the detailed guide by VoIP.ms.