The SIP server for the Zipatiles is a good system for integrating with various intercoms, and between another Zipatile or group of Zipatiles.
For setting up a SIP server you will need at least 1 Pro licence. This Zipatile with the Pro licence will be the device that hosts the SIP server.
Any other Zipatiles simply connect to this Zipatiles SIP via the Zipato Intercom app and do not need a Pro Licence for this. They will automatically Register themselves when on the same network.
For help in setting up the SIP server, check out Zipatos Youtube channel here. https://www.youtube.com/user/zipatotv/search?query=sip
Also have a look at this step by step guide from Zipato for setting up your SIP with 2N Helios, Akuvox R27 and VBell/AVA Design Intercoms and the Grandstream Handsets.