Incidentally, telephone taps are illegal and we do not recommend breaking the law. There are plenty of legal tools out there and we will explain some of them. Your silent partner should be very dependable. Choose equipment designed for the purpose. If for example you wish to monitor an area of your business to confirm an abusive situation, you can have a camera put in and risk everyone knowing by seeing the wiring put in.
Wired systems work and are dependable.
They aren't portable, but some situations remain the same and a good quality wired system is tops. Have it installed during a remodel. A good quality wireless system costs just a little more but you can avoid installation costs as units are as easy to install as any clock radio, table lamp or the wall clock. Our products work completely in every way. The only difference is that ours, sends a video signal up to the specified distance with high fidelity.