On a time when everyone calls banks and credit card operators via cell phones and wireless phones at home,VOIP is about as safe as any other phone.
Yes, you could decode the whole internet, and sniff the packets, and find out conversations on VOIP, and most likely will be someone calling his or her grandmother overseas, or my daughters calling friends out of state.
The chances to get a important or financially good information are really small.
But if you need to protect a phone call for real, make a data VPN end to end,encript it, and pass a VOIP call trought the tunnel.
Will take a lot ot money and time to crack smoething like that.
Will be cheaper and easier just to break on your house or office and put a transmitter on the phone itself.
Best regards.