Google voice also acts as a proxy for inbound calls. Your friend dials your GV number, GV dials your other phone number, which does not belong to Google and use

Google voice also acts as a proxy for inbound calls. Your friend dials your GV number, GV dials your other phone number, which does not belong to Google and uses your friend's number (which may or may not belong to Google) as the caller ID. From the point of view of your telco, who services your phone number, Google is spoofing your friend's number unless your friend happened to call using another GV number.