The reason Su Xiao asked the Old Priest to help contact the Einzbern Family was mainly due to the Old Priest's special status.
If Su Xiao were to contact the Einzbern Family himself, they would likely not respond, let alone reach out to Emiya Kiritsugu, who was participating in the Holy Grail War through the Einzbern Family.
The Old Priest was the Supervisor of the Fourth Holy Grail War, and his words carried weight. The Einzbern Family could not ignore his requests. Su Xiao did this to try to pull in other combat strengths, in order to deal with those three High-Tier Contractors.
The time agreed with Umbrella Brother was approaching; after leaving a string of phone numbers for the Old Priest, Su Xiao left the church.
...
Night, one kilometer outside of Fuyuki City, in the forest.
An owl perched on a tree branch, tilting its head to observe a human below, the scent of blood on him evoking its wildness.
"Hoo, hoo, hoo..."
