Who calls Reverend Hale to Salem?

Reverend Hale of Beverly is mentioned several times by the townsfolk before he makes his appearance in the middle of act 1. It is obvious by the references that Rev. Hale carries some authority on witchcraft and has experience with investigating witches.

PUTNAM, as though for further details: They say you've sent for Reverend Hale of Beverly?
PARRIS, with dwindling conviction now: A precaution only. He has much experience in all demonic arts, and I—
MRS. PUTNAM: He has indeed; and found a witch in Beverly last year, and let you remember that. (act 1)

The people (Mr. & Mrs. Putnam mainly, at this point) who want to hunt and find witches assume, since Rev. Hale has been called, a witch must be present. It never occurs to them that his expertise will confirm there is NOT a witch in Salem, despite Rev. Hale mentioning this outcome as a possibility almost immediately after his entrance:

PUTNAM: She cannot bear to hear the Lord's name, Mr. Hale; that's a sure sign of witchcraft afloat.
HALE, holding up his hands: No, no. Now let me instruct you. We cannot look to superstition in this. The Devil is precise; the marks of his presence are definite as stone, and I must tell you all that I shall not proceed unless you are prepared to believe me if I should find no bruise of Hell upon her. (act 1)

Rev. Parris is the one who called for Rev. Hale of Beverly to come to Salem:

PARRIS, his eyes going wide: No—no. There be no unnatural cause here. Tell him I have sent for Reverend Hale of Beverly, and Mr. Hale will surely confirm that. Let him look to medicine and put out all thought of unnatural causes here. There be none. (act 1)

Parris is convinced most people in the town are persecuting him for some reason. He sees the suspicion of his daughter Betty and his niece Abigail as witches to be the reason his persecutors will drive him from Salem. Rev. Parris has hopes that Hale can prove there is no witchcraft in Salem, especially involving his daughter and niece. Despite Hale being called to confirm there are no witches, his very presence helps to drive the hysteria that causes the witch hunts.