Reference9:Concept Tones on Disconnect and Release from remote

When a call on a phone is terminated from remote (that is, the other side hangs up), the call will not be terminated immediately. Instead, tones are played for a while. This is done so that the non-terminating party realizes that the call was terminated deliberately (by the remote party). However, in certain scenarios, this is neither required nor desired, so here is how it works in detail:

Release before connect - internal & external calls

the call is terminated before it is connected

• handset mode -> Busy tone until user goes on-hook
• hands-free mode -> Busy Tone for 30 seconds
• headset mode -> Busy Tone for 30 seconds

Release after connect - internal call

the remote end was local to this PBX system (internal) and the call was connected before it is terminated

• handset mode -> Busy tone until user goes on-hook
• hands-free mode -> Busy Tone for 2 seconds
• headset mode -> Busy Tone for 2 seconds

Release after connect - external call, no in-band info

the remote party was external to this PBX system and the call was connected before it is terminated. The remote end does not indicate that there is still in-band information (e.g. a busy-tone) available

• handset mode -> Busy tone until user goes on-hook
• hands-free mode -> Busy Tone for 2 seconds
• headset mode -> Busy Tone for 2 seconds

Release after connect - external call, in-band info indicated

the remote party was external to this PBX system and the call was connected before it is terminated. The remote end does indicate that there is still in-band information (e.g. a busy-tone) available

• handset mode -> in-band info for 30 seconds, then Busy tone until user goes on-hook
• hands-free mode -> in-band info for 30 seconds
• headset mode -> in-band info for 30 seconds

Signalling of in-band information available is unreliable. Some providers indicate but do not provide in-band info and thus annoying tones may be heard for a long time. To prevent this the Automatic Hangup check mark may be set on the PBX Trunk Object which will release the call 2 seconds after disconnect.

?? what happens if the endpoint is under SOAP?

?? How is internal/external detected?

Internal class are detected from:

Inbound: Setup - Information Element: Calling party number: Numbering plan: Private numbering (0x09)
Outbound: Connect - Information element: Connected number: Numbering plan: Private numbering (0x09)

All other calls are assumed to be external, i.e. before connect the type of an outbound call is not known in the phone.

?? which will terminate the call 2 seconds after does not fully explain the phone's behaviour in this situation

The phone first sees a disconnect(in-band-info) and plays what's received from the network.
If Automatic Hangup is checked it will see a release 2 seconds later and because a diconnect was seen before it will not play any tone.   

?? does all that above also apply if the call is terminated using a release right away (instead of a disconnect)?

Yes, on a release without a preceeding disconnect the tones are played by the phone although the call itself does not exist anymore.