Here's the sequence I'm seeing with a packet capture from the Asterisk server:
-	301 calls 302.  The INVITE from 301 hits Asterisk.  Asterisk sends back 100 Trying, and then a NOTIFY to 302 and 303 (which are both subscribed to 301, and they see 301 change to “in use”).  302 and 303 send back 200 OK for the NOTIFYs.
-	Asterisk sends the INVITE to 302.  302 sends back 100 Trying and then 180 Ringing.  After the 180 Ringing, Asterisk sends a NOTIFY to 301 (which I have subscribed to hint 305, which is mapped to SIP/<device> corresponding to the one I have logged in as 302).  No other NOTIFY updates are sent at this time.
-	I pick up on 302.  302 sends a 200 OK with session description to Asterisk.  Asterisk ACKs this 200 and then sends 2 NOTIFYs to 301 (which is subscribed to both the 302 “Custom device” hint and the 305 “SIP Physical device” hint), and also sends 1 NOTIFY to 303 (which is only subscribed to the 302 “Custom device” hint).
-	While 301 and 302 are still “on the call”, 303 calls 302.  The INVITE from 303 hits Asterisk.  Asterisk sends back 100 Trying, and then a NOTIFY to 302 (subscribed to the “Custom device” hint for 303) and 2 NOTIFYs to 301 (subscribed to both the “Custom device” and “SIP Physical device” hints).
-	Asterisk sends the INVITE to 302.  302 sends back 100 Trying and then 180 Ringing.  After the 180 Ringing, Asterisk sends a NOTIFY to 301 (which updates the “SIP Physical device” hint).  No other NOTIFY updates are sent at this time.
-	303 cancels the call and sends a CANCEL to Asterisk.  Asterisk responds with 200 OK, sends a NOTIFY to 301, sends a CANCEL to 302, and then sends another NOTIFY to 301.  303 then ACKs Asterisk’s SIP 487, and Asterisk sends 1 additional NOTIFY to 302 and 301.
-	301 and 302 finish their call.  Asterisk sends a total of 1 NOTIFY to 302 (the status for “Custom device” 301), 2 NOTIFYs to 303 (the status for “Custom device” 301 and 302), and 2 NOTIFYs to 301 (the status for “Custom device” 302 and “Physical Device” 302).

So, it looks to me like I’m missing NOTIFYs for RINGING or RING_INUSE events on Custom devices.

Verbose 3 output is attached, the short of the output is – there is no console output showing == Extension Changed 302[hints] new state on the Ringing or InUse&Ringing events – only on InUse or Idle events (which matches what I’m seeing on the phones).