Newer devices are usually able to register a keep alive task with the modem directly, this allows the device to go into deep sleep like normal, whilst the modem (which is always on anyway) handles pinging the SIP end point to keep the connection alive. I think with Qualcomm chips and supporting software several SIP tasks are handed off to the modem, this is required now to support VoLTE and VoWiFi.
If for some reason the keep alive task can't be handed off to the modem (they will only handle so many apps handing off these sorts of tasks, or it might not be supported by the SoC of the device, or the software doesn't do it) it means the main CPU remains powered-up, or is made to keep waking up, and that immediately hits battery life.