I did test at my work, I have got a 2600 router with external modem connected to the router via auxiliary port (not serial interface),in one department, and My laptop with internal modem in another department.
I tried to access the 2600 router via laptops modem , I was able to access the router.
But if the routers modem stayed idle for 15 minutes it disconnected the session.
Do I need to add additional configuration (script) in order to prevent this disconnection ?
I configured routers modem like this:
Router2600 # configure terminal
Router2600 (config)# line 65
Router2600 (config-terminal)# modem autocnfigure discovery