Best practice here is giving specific meeting bodies a pin free number by assigning that number to the meeting series or project that houses their meetings. By doing this, they can promote the same no-pin dial in number for all of their meetings.
You can setup the pin free number in the dashboard in the Settings → Phone Lines tab:
You can now head to your project or meeting body to see that the number has been assigned. Dialing this number will now route to the default phone tree for this project, i.e. voicemail box or meeting menu.
Within a 30-minute buffer of any given meeting on the assigned meeting body or project, this number will present the option to listen to the meeting or leave a recorded message for the meeting.