Ring Group with single extension sends INVITE back to trunk

Bizarre behaviour that causes problems with one trunk provider.

If there is only one extension in a ring group, when an incoming call is sent to the group the dial plan generates an INVITE to the trunk the call is coming from.

Some providers discard / disregard it, one drops the incoming call with a "Forbidden" (rightfully).

If there are two extensions, and ring strategy is ringall all works OK, no INVITE is sent to the trunk.

If there are two or more extensions and the Ring Group calls just one extension (e.g. random, memoryhunt, etc) the same error happens.

Why would the Ring Group send an INVITE to the trunk in the first place?

Please note the Ring Group is set with "Call Progress" NO and the trunk is set to canreinvite=no.

And before you ask, Ring Groups with a single extension are the only practical way I can see to create a call distribution pattern not using a Follow Me.

Follow Me is not suitable when you need independent destinations for different incoming call sources.

And before you ask again, creating a Virtual Extension and assigning it to the Ring Group doesn't change the behaviour. Same error, as the Virtual Extension is not rung.

The only way out I had was to create and assign a second SIP extension on an ATA that doesn't have a PSTN handset connected, add it in the Ring Group and use a ring strategy that calls both.

From my point of view it's a bug in the dialplan generated by the Ring Group module.