Fundamentally, broadcasting static during a call is a sign of the fact that signal power is degrading (or that there is no sound coming through whatsoever). When signal strength degrades sufficiently, the static resonance emerges. 

When there is no communication coming through, it’s a slightly separate story. A walkie talkie has what’s renowned as a ‘squelch’ monitor circuit that keeps tabs on the signal strength. The squelch circuit will mute the amplifier when it realizes that there’s no signal coming through into the device. This is, fundamentally, the exact same purpose as your Television has as soon as it cuts off an unavailable channel after a fixed time. However, in the moments before your 2 way radio ‘squelches’ the signal, you’ll hear static, or ‘white noise’ as it is also called. 

‘Squelching’ is a fairly vital part of any/all broadcasting gear. The method used in your 2 way radio known as a ‘carrier squelch’ is more likely to be manually adjustable. 

From Wikipedia (as of may 2013): 

“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’s ‘white noise?’ In accordance to Joe Shambro, writing for About.com’s handbook to home 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 sound degradation on your 2 way radio, there may be a number of causes for this. ‘Wireless Woman’ a writer with an excellent website about two way radios, has this to state:

“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 here you have it.