The FreedomPop Global SIMs should still be available directly from FreedomPop. They're actually Three UK SIMs, which roam on HSPA outside the UK; FreedomPop's version is also blocked from the LTE network in the UK.

They're... usable. Your data all routes through the UK, which isn't much of a performance hit in France but makes for some pretty ridiculous latency in the US. For the price (free, as long as you turn off all the 'free trial' add-ons), they're worth keeping in your bag for short trips or to send a quick e-mail before you can grab a local SIM. They used to be great for European travelers, but now that nearly every European SIM includes free roaming in the EEA, they're not as useful over here.

To answer OP's question: get a local SIM and use Google Voice with the Hangouts dialer. Free calls to the US and Canada--completely free on wifi, or the cost of the data if using the cell network.