Originally Posted by UKJack

it is extremely easy to listen to voip calls I have done so on many occations (just for fun though) If you want to prevent this from happening do not use a wireless router and make sure the phone is not wireless.

if someone is doing this to you chances are he is using cain http://www.oxid.it/cain.html

as it the easiest program to use for voip interception.
If you want to find out for sure speak to a friend using voip and talk a load of shit make it sound real though and if he comes back to you with that info then you will know for sure