In June I got Sprint to release a device I bought on eBay that I never used with Sprint. There was a customer service number online for Sprint that I called, and they released the device IMEI so I could activate with a new Visible account. Good luck.
I also had an issue porting from Xfinity as well. I was trying to port using the Visible app (using iPhone) but whenever I clicked 'Confirm' to continue I kept receiving an error saying "Select Carrier". Oddly enough, I wasn't getting this error when I clicked on a different carrier other than Xfinity.
Solution: I had to contact Visible support and have an agent conduct the port for me. Just needed to provide the agent with my information through the support chat and it was completed after a few hours.