These listening devices can be planted any where within walls, phone, fax, computers lines, etc. and then transmit the signals to another location.
Since these devices are continually upgrading, keeping up with the latest technology and equipment to locate these bugs (transmitters) is essential.
As a reminder, do not use any hand held mobile phone when having any confidential private phone conversations. Simple scanners can be used to intercept these signals (2.4GHZ) and your conversation will be overheard. It is still recommended that the basic landline phone be used for these types of private calls.