There is a 'queue no answer' option, however not sure if it applies to a trunk other than 'straight' SIP type - or a PSTN trunk throuhg a (SIP?) gateway...
(and it's a /queue/ option, not available in a ring group; also the queue can be set to have MOH class 'ring only', and a 'strict join empty' so the callers won't be allowed to the queue when there are no extensions registered in it).
Queue also actually solves the 'simultaneous incoming ringing' problem, which the ring group does not.