FireRTC allows users to change their caller ID. I usually go get a US number from a website that generates one whenever a fucker blocks my old caller ID. You can change your caller ID by going to the Settings Tab.
There is another service called Google Hangouts. You are able to use that too if you want to call US numbers. I am unable to provide any info on this service since I never had the interest to do so, as long as I have been only using FireRTC just fine.
Just my two cents on spoof calling.