If you place the Intercom call with your handset on the cradle, it will be a ringing intercom call. If you lift the handset, press ITCM, and call the other extension, it will be a voice call.
After you place an Intercom call, pressing # will toggle from voice to ring and back to voice again.
You can also (sometimes accidently) dial a feature code to block voice announced calls. I would have to look it up, unless someone else knows it offhand. Sometimes the code is similar to the beginning of a local phone number, and gets accidently dialed on the Intercom instead of an outside line.