Essentially, broadcasting static during a call on is a sign of the fact that signal power is degrading (or that there’s no sound coming through in any way). When signal power degrades sufficiently, the static sound emerges. 

When there’s no communication coming through, it is a complete different story. A walkie talkie has what’s acknowledged as a ‘squelch’ monitor circuit that keeps tabs on the signal power. The squelch circuit will mute the amplifier as soon as it realizes that there’s no signal coming through into the radio. This is, fundamentally, a similar function as your Television has when it cuts off an unavailable channel after a set time. However, in the moments before your two way radio ‘squelches’ the signal, you will hear static, or ‘white noise’ as it is also known as. 

‘Squelching’ is a fairly vital a part of any/all broadcasting gear. The method utilised in your walkie talkie is called a ‘carrier squelch’ and is more than likely to be manually changeable. 

From Wikipedia (as of 17/5/13): 

“A carrier squelch or noise squelch is the most simple variant of all. It operates strictly on the signal strength, such as when a television mutes the audio or blanks the video on “empty” channels, or when a walkie talkie mutes the audio when no signal is present. In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in base stations at remote mountain top sites are usually not adjustable remotely from the control point.

In devices such as two-way radios (also known as radiotelephones), the squelch can be adjusted with a knob, others have push buttons or a sequence of button presses. This setting adjusts the threshold at which signals will open (un-mute) the audio channel. Backing off the control will turn on the audio, and the operator will hear white noise (also called “static” or squelch noise) if there is no signal present”. 

So what exactly is ‘white noise?’ In accordance to Joe Shambro, writing for About.com’s guide to household recording, 

“White noise is a static sound that has equal energy on every frequency. Think about this for a second: every frequency from 20Hz to 20kHz is equally represented at the same velocity; this type of frequency scale is called a “linear” scale. This gives the noise a uniform, static sound that the human ear detects as somewhat harsh and heavy-handed toward the high frequencies. However, white noise represents a very unnatural way of presenting frequency data in terms of how our ears work.” 

 If you are experiencing signal degradation on your two way radio, there may be numerous causes for this. ‘Wireless Woman’ a writer with an brilliant website about two way radios, has this to say:

“Signal loss can happen in any number of places in the system. Antennas may not properly direct the signal toward the horizon. Cables to repeaters may need replacement. Connectors can be corroded. Finally, the quality of the system may not be adequate. The old adage “you get what you pay for” certainly applies to two-way radios, and their quality does vary. Receiver specifications, engineering, tuning, the antenna, and even design generally improve as the price increases. This really happened: Two public safety officers were at a scene. One was using a radio that cost more that twice as much as the other. Guess which one could hear and which one couldn’t?” 

So there you have it.