Q:

Is there a SIP/VOIP-agnostic way to know when a phone goes offline?

Working on a SIP application that requires monitoring call activity/state on a phone. My app (UA) is registered with a proxy server, as is the phone being monitored. I am making use of SUBSCRIBE/NOTIFY messages to tell the phone that I want to know about its "dialog" events, and once subscribed, this works fine. I am running into two problems:

I am requesting a subscription length of 900 seconds and consistently getting some absurd amount of 4000+ seconds returned from the phone.
There is the possibility that the phone may be unplugged while the subscription is active (don't ask, but, yes, this is a possibility). When this happens, I am not getting any type of notification as to the phone no longer being available. Consequently, I have to wait for my subscription to time out, try and renew it, handle the failure, and create a new subscription.

I realize that I can renew my subscription sooner than its expiration time, but is there a better way to go about it? For example, would I be able to get "presence" events and use them to know when the phone is there or not? Are those events always guaranteed to happen on phone start-up, and even so, since there is a proxy in the middle, would I even get the notifications, or would I still not know that the phone restarted and no longer recognizes my subscription(s)? Are these "presence" updates something that I can request from the proxy?
Any insight would be great; for the moment, I'm going to simply try and refresh my subscription on a much smaller interval, but I would love to know if there is a better way to go about it. Thanks in advance.

A:

I don't know about a SIP agnostic way other than perhaps somehow pinging the phone but there is definitely a SIP way to do presence and that's using the same subscribe/notify model you are using for dialogs but in this case for presence. The relevant standard is RFC 3856 A Presence Event Package for the Session Initiation Protocol (SIP).
Typically the presence notifications would be generated by a SIP registrar, and since you say your phone is registering with your proxy it must also have a registrar built in. The registrar would notify your UA when the other phone's registration status changes such as when it first came online or when it failed to re-register. As far as the delay between a phone "disappearing" and the registrar server detecting it there's no prescribed way to do it. You can either wait for the registration period to expire and mark the phone as offline when it fails to register or alternatively the server could actively send a packet or even a SIP request such as an OPTIONS request to the phone and mark it as offline if it got a failure response, Asterisk uses te latter approach.

