This isn't an answer from the home network -- it's a routing failure (DIAMETER_UNABLE_TO_DELIVER, RFC 6733). The node that generated this response couldn't find any host in the destination realm able to handle the request, so the request never actually reached the home HSS to be evaluated.
Where you see it
In the Result-Code or Experimental-Result-Code AVP of a Diameter answer (S6a, S6d, Gx, Gy, Cx…).
What to check
Check which node generated this response (its hostname is in the message) -- if it's an intermediate routing/interconnect agent rather than the home HSS, this is a routing/interconnect problem to raise with whoever operates that agent, not something the home network ever got a chance to reject or accept.
Look at the Origin-Host of the answer: an intermediate agent (DRA/IPX) and the home HSS mean different owners of the problem.
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