How many telephones can I have in an installation?
The only limit you must respect is that of not installing more than 4 telephones in the same house or flat, i.e. associated with the same call wire.

 What do we mean when we say that the door entry system has privacy of conversation?
The privacy of conversation means that no two houses or flats can communicate with the entrance panel at the same time. This ensures that the conversation between a given dwelling and a caller remains completely private.

 Can I replace the telephone of the electronic door entry system of my house/flat with an ALCAD telephone?
First of all, you must make sure that your telephone uses the 4+N system. If it does, you can replace it with the TUN-001 universal telephone. The compatibility table which comes with the TUN-001 will indicate the terminals corresponding to the model you wish to replace.

 Can I connect any electric lock available on the market?
You can directly connect any 12 Vac electric lock if its consumption does not exceed 800 mA.

 In a factory, access is controlled by a system using a keypad. It is decided that an electronic door entry system should be installed as well and that both systems should operate the same electric lock. Is this possible?
For this type of installation, use the APC-001 accessory, which will allow you to operate the electric lock from either of the two systems. Moreover, you will be able to activate electric locks using alternating or direct current.

 I have a door entry installation with 2 points of entry. At each point of entry, there is an entrance panel. I have used a single power supply unit to feed the two entrance panels. I am unable to determine the cause of a buzzing noise in the audio system.
The buzzing is due to your having used the same power supply unit for both points of entry. Use a different power supply unit for each point of entry.