Because a SIP client software is installed in the Raspberry Pi is set in relation to the SIP client software on the tablet or Android Smartphone when a visitor presses the call button. The linkage between the two is made by the Open Source Asterisk SIP server that serves as PBX.
The advantage of this technology is that it is Open Source and thus we can make devices together from different sources operate, use smartphones or tablets or even PC to communicate. Just install a SIP client. For the door opening on selected parameters in the system, eg dialing tone dialing (DTMF).