What could be the problem with iOS+VPN+SIP?

Question

iOS

There is s1: ipsec+ikev2 vpn is configured via certificates
There is s2: sip telephony (kamailio) is configured
There are several iphones with the latest ios, Grandstream Wave and Bria are used as clients for sip.
On iphons, vpn is configured via .mobileconfig with "VPN on-demand" for all types of networks.
For about a month, the following problem began: the iphone is blocked, after a few minutes (5-7), notifications from sip, mail stop working until the moment you pick up the device again and unlock it. Apple pushes from other apps are fine.
From the s2 side, all clients hang as if online, from the s1 side, there is also the appearance that they are alive. I tried to play with the keep-alive, delay and other parameters, but did not achieve success.
Nobody faced such a problem?