It's possible.
But it's not as easy as just changing a parameter in the DTMFOUT statement.

If the keypad is using ASM interrupts (not ON INTERRUPT), (RBC, portb change).

Then when a key is pressed, the handler should set a flag that tells the main loop to start the DTMF with the Maximum ON time (65 seconds).

When the key is released, the handler will simply set the system variable R1 = $FFFF.
The DTMF command will then terminate.
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