I use a Kenwood TS-480HX for my remote station. The free Kenwood rig control software for the TS-480 has a menu item that allows you to set a transmit time after which the radio will shut down. It doesn't turn the power supply off however. I believe I set my timeout for 20 minutes. If the station loses the internet connection while in the transmit mode it will terminate the transmission 20 minutes after the start of the transmission.
Which is not inline with remotely controlled station rules...if you lose control, the station is supposed to terminate transmission in 3 mins....not 20...see Part 97.213(b) Provisions are incorporated to limit transmission by the station to a period of no more than 3 minutes in the event of malfunction in the control link.
In internet remote control, best to have a watchdog timer that pulses the box at the remote site every two mins...if the box fails to see a simple ping or whatever, terminate transmitter..
Best thing to do for dialup is monitor loop current or listen for dialtone..or look for loss of line voltage in CPC (Calling Party disconnect Control)...if it sees it go away, cut the transmitter....radio station monitor dial up boxes use this all the time..so do answer machines (for those who dont use voicemail)...its a 300ms loss of voltage and current on the line when the distant party hangs up.....however, if the Central Office screws up, the line could stay locked on...
best to have dial up for backup to the internet or a UHF rcvr so someone local can terminate it for you