Is it "isayi" as in followers of 'isa spelled with an initial 'ayn? Now that's interesting, since i thought that no christian would refer to jesus as 'isa, since afaik that form of the name can only be traced back to the quran. That would suggest that the christian population among urdu speaking people were initially muslims, then missionaries would use the term for jesus that they were already familiar with, ie the term they knew from islam, 'isa. And i guess the word stuck with them. Was such the case?
Basically, all of the religious vocabulary in Urdu is taken from Arabic, for both Christians and Muslims. Desi Urdu-speaking Christians draw on Christian Arabic for their religious vocabulary. It is interesting to note, however, that Jesus is Christian Arabic is not 'isa but rather yasuu' (much closer to the Hebrew original, yeshuu', which is BTW the same name as "Joshua" in Hebrew). What seems to have happened is that Christians in India simply took the best known name for Jesus, 'isa, as made famous by centuries of Islamic civilization, for their own. Christians in India and Pakistan call Jesus "isa masiih" (as opposed to Arabic Christians, who call him yesuu' al-masiiH). For reference here are all the spellings in Arabic characters:
عيسى مسيح=isa masiih
يسوع المسيح=yasuu' al-masiiH