Rights to Assets in Living Trust?

My aunt created a living trust before she died that left all her assets to her adult children, Phil and Joe. Both Phil and Joe were co-trustees of the trust. Phil passed away without a will 3 months after this occurred.

Then 3 months later, Joe passed away, with a will that names me executor. I have been named as a new trustee of the trust and need to transfer the assets to the estate of Joe to distribute the assets. However, there is a dispute over whether Phil's share went to Joe upon his death, or if Phil's share of the trust would go to his father...

thoughts?