|I've never heard of a "leased line modem"|
If you're considering a leased line from an internet provider, they'll provide all the equipment necessary to create the leased line. You would simply have to connect each end to a device in your network........like a router or switch.
Typically, they place an "edge device" at each location which you connect your network to. It could be a router, or a switch. Normally those devices will also have a dialup modem connected to them so the provider can remote in and check their edge device should it begin to misbehave.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.