Hi Anthony,
From our experience
Zoiper is the best softphone option. You will want to use the RingRoost "SIP Phone" control to register your Zoiper Softphone so you can start making/receiving calls.
The process for registering a softphone (virtual phone) is pretty much the same as registering a hardphone. See here: http://www.ringroost.com/how-to-set-up-and-register-a-sip-phone.php