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

When there’s no communication coming through, it is a complete separate story. A walkie talkie has what is acknowledged as a ‘squelch’ control circuit that maintains tabs on the signal strength. The squelch circuit will mute the speaker when it realizes that there’s no signal coming through to the radio. That is, essentially, a similar purpose as your TV has when it cuts off an unavailable network after a preset time. However, within the moments before your two way radio ‘squelches’ the sound, you will hear static, or ‘white noise’ as it is also referred to as.