Some phones glow to indicate when you have voice mail messages waiting. The number of messages waiting can be controlled by a dev specific command. The IP Office server or other IP Office applications might also control the message waiting lamp. Send the following buffer to lineDevSpecific:

unsigned char buf[21];
int len = 21;
buf[0] = 9; // Shortcode
buf[1] = 73; // Set MWL
sprintf(&(buffer[2]), ";Mailbox Msgs=%d", num);
// Where num is the number of messages