Currently the Incoming Call Routing notification rule does not offer voice as a notification option. This is because voice notifications for the alert could potentially interfere with the routing of the live incoming call (i.e. if an incoming call was forwarded to a user, and this user also got a voice notification for the alert generated from the incoming call)
We could add voice as an option for incoming call routing notification rule, and potentially require it to be added with at least a 1 minute delay to avoid interference with routing of live call
You can add the team/escalation/schedule as a responder to incoming call alerts (through advanced settings of incoming call integration) so that users are also notified thorugh their "New Alert" notification rule, which supports voice notifications.
However this workaround can result in duplicate notifications, and will not respect the "notify only if voicemail is left" setting.