You can check the up-to-date list of all Acrobits Push (SIPIS) servers at any time by resolving the all.sipis.acrobits.cz DNS name. The current list of IPs (both IPv4 and IPv6) follows:
159.65.167.207
159.65.186.176
159.65.251.173
159.65.252.186
159.65.253.49
159.89.179.103
162.243.226.164
165.227.65.164
165.227.115.186
165.227.182.9
165.227.184.188
165.227.190.186
165.227.210.221
165.227.223.68
167.99.48.91
167.99.119.203
167.99.119.244
2604:a880:800:10::160:1
2604:a880:0:1010::d31:1001
2604:a880:800:a1::172e:b001
2604:a880:800:10::69d:5001
2604:a880:800:a1::1ad:1
2604:a880:800:10::7e:d001
2604:a880:800:10::790:4001
2604:a880:800:a1::1777:f001
2604:a880:800:10::292:1
2604:a880:800:10::5db:1
2604:a880:800:a1::176e:5001
2604:a880:800:a1::1777:e001
2604:a880:800:a1::176e:3001
2604:a880:800:a1::172b:7001
2604:a880:800:a1::174f:e001
2604:a880:800:10::62b:1001
2604:a880:800:10::72d:b001
These IPs need to be able to communicate with the SIP registrar (PBX) server over whatever port is used for SIP traffic (typically 5060 or 5061 for TLS).
The user devices need to connect to these IPs on the following ports:
443/TCP
4998/UDP
4998/TCP
24998/TCP
The user devices also need to connect to the following IPs on port 443:
165.227.103.7 (reg.acrobits.cz)
2604:a880:800:a1::12f9:c001 (reg.acrobits.cz IPv6)