Google "APNS" and do a bit of reading on how it works with resource management on iOS, that will answer your question. You can also google: "jabber APNS" for specifics of how it's implemented in Jabber.
If they do not logout from Jabber, APNS should make the devices ring, if that is not the case, then you need to review your APNS configuration and troubleshoot as necessary.
HTH
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