There are a number of fake Fermax Citymax phones sold on market places in Singapore for cheaper price. Which in many cases will not work with your existing system. We can have a look at one of them:

There reason why fake Fermax fails to work on certain Fermax installation is mostly because of volume control. Even 4+n system is mostly original it still has some service signals between the equipment which is not heard during normal operation. When impedance of the set is higher compare to the original set you may not be able to receive the call from the door station. Tested before.

I also have fake intercom which looks like normal Fermax Citymax however it is completely different inside. It also cannot work at all.

Lol. Another smart guy bought Fermax Loft 3390 (VDS) instead of Loft 3393 (4+). Lucky not from my shop. I don’t think he will get his money back. See the difference between Loft 4+n and Loft VDS on respective page.