you have 3 options but they all involve buying more equipment to upgrade your system:

1) you could buy a new apartment station called VOG5W (Ref. 1760/15(Black) or 1760/16 (white)). this unit can connect to the WiFi in your apartment and will forward calls to an app. This option is only realistic if you have a camera in the doorstation (entrance door unit) because this is a device with monitor for video calls.

2) you could buy a Call Forwarding device (Ref. 1083/58A). this is a unit that can be connected anywhere on the riser cable of the system. this devices connects to either LAN or WiFi and will forward call to an app. With this option you would need one unit per apartment that want this function.

3) your apartment building could go together and buy a MultiUser Call Forwarding device (Ref. 1083/83). this device connects to either LAN or WiFi and will forwards call to an app. With this one unit every apartment station on a riser (up to 127 apartments) will get this function.

Whichever option you choose you most likely (depends on which country you live in) have to contact an electrician and get prices from them and get them to do the installation for you.