Here is the list of S0-20 screen icons and their meanings.
Network Alert: indicates one or more subscription service (such as voicemail) is going to expire soon.
Ringer Off: indicates the ringer sound is turned off. To turn the ringer on or off, long-press the "#" key.
Microphone Mute: indicates the sound input is muted (during the call). To mute or unmute it, during a call select Option > Unmute from the menu.
Missed calls: indicates there are new missed calls. To view the details, go to Menu > History > Missed Calls.
New request: indicates there are new contact requests. To view the details, go to Menu > History > Contact Requests.
New Voicemail: indicates you have new voicemails. To listen to them, go to Menu > History > Voicemails.