In this site, you will read inquiries and answers if your phone is being wiretapped. The was topic posted on with a non-Vonage affilited Vonage Forum. A member started the thread asking how do we know and check if phone is being wiretapped.He then exclaimed that some devices exist that can detect drop in line voltage.
But he added if you're truly concerned of being wiretapped, he said not to use a cordless phone. Use the corded one and for some certain reason, there will be no way to wiretapped you. Visit the site for more info.
Vonage Forum Expert: How to avoid being wiretapped | IP Telephony, VoIP, Broadband