Hi- I'm in the same place, and I couldn't find a proper integration. So, I built my own click-to-dial.
I only needed two things- a call centre definition file for SFDC to activate their click-to-dial fields, and a visualforce page that contains the javascript to poke the Zoiper client, using the OpenCTI demo code.
I did add a bit of extra Javascript to check it's a valid phone number, and open the SFDC "Log a call" window. Rather than use the proper API, I just use the callto:// protocol handler for simplicity, but it does make click-to-dial work, and an actual developer could do a lot more.
Happy to share the code if you want to try.