MK 160 ON/OFF-mode doesn't work

I soldered two kits MK 160 for to switch on a heating by two telephone-calls in the row.
Unfortunately the relay doesn’t react, although the LED is lightning during the two calls (ON/OFF mode).
When I choose the other mode (ON with auto turn-off timer) the kit works in the right way after one call.
What may be the mistake, that both kits don’t work in the ON/OFF mode?
Thank you for your advices!

Make sure the time between two calls and the duration of the call is correct. A too long or too short time will not trigger the unit. Make sure that the backlight of the cellphone is continuously lit during the call ie that it is not flashing.

Unfortunately the problem still remains: The unit cannot be triggered in the ON/OFF-mode, no matter how long the time dures between the two calls in the row. It also makes no difference, if the backlight of a cellphone is lit or a lamp.
Only the ON-mode (with auto turn-off timer) can trigger the relay. Do you have another idea how to trigger the ON/OFF-mode?

Sorry, at this time we have no clue as to what the reason might be. We can offer inspection and repair if you return the unit.