SIP client alternative to Facetime? Wikipedia says:

"Like the video version, FaceTime Audio is currently only available between Apple devices."

So...I was going to ask how to make/receive SIP calls from Facetime...but that's not an option.

what's a free SIP client for Mac OS x laptops?
I'd start with Telephone from the App Store and then once you have some more detailed requirements, you could refine what you're looking for.

https://itunes.apple.com/us/app/telephone/id406825478?mt=12