The latest seem to be terminal servers (e.g. Oracle Secure Global Desktop) with time managed access to certain services.
Vendor calls customer, customer grants access for a specific user and system for a limited time, all on screen actions are recorded.
In the past more funny things existed. Some vendor support was by modem dial in only (e.g. EMC), so the customers had a switch that connected/disconnected the physical phone lines to prevent unwanted access.