You could look at this from the extreme end of the scale, and assume that NO communications, aside from standing in the woods whispering to somebody, is truly secure. Or, you could look at the "commercially reasonable" standard: landline (POTS) telephone service, and VoIP calls that traverse it, are not encrypted. OBi devices use the Google Chat/XMPP protocol, not Google Voice, per se. Those calls are not encrypted.
Mobile phone calls are encrypted, but that encryption can be broken, with specialized equipment. Some law enforcement agencies have equipment ("stingers") which can trick mobile phones into registering with their fake "cell tower" instead of the carrier's towers, and thus, capture call information.
The chances of your personal information leaking via other sources are much higher than someone "tapping" or intercepting your telephone calls, e.g. retail store POS systems, businesses that store your personal information getting hacked, etc. Reputable business will never ask for your entire social security number over the phone, just the last 4 digits.