You had it assigned to the voicemail hunt group. The system calls up the first extension in the group every 30 minutes to sync the time and date, and everytime you press the Night Service button. The touch tones you heard are "mode codes" that communicate from the ACS to the voicemail. The intercom sounds like outside dial tone because some outboard voicemails "listen" for dial tone before dialing, and are confused by internal dial tone.