"users" : {
    "userId1" : {
        "name" : "Alex",
        "email" : "alex@gmail.com",
        "phoneNumber" : "123456789"
    },
    "userId2" : {
        "name" : "Ben",
        "email" : "ben@gmail.com",
        "phoneNumber" : "223456789"

    },
    "userId3" : {
        "name" : "Charles",
        "email" : "charles@gmail.com",
        "phoneNumber" : "323456789"

    }
}

self.ref = FIRDatabase.database().reference()
    ref.child("users").child("userID").observeSingleEventOfType(.Value, withBlock: { (snapshot) in

        if snapshot.hasChild("phoneNumber"){

            print("Phone number exist")

        }else{

            print("Phone number doesn't exist")
        }


    })

FIRDatabase.database().reference().child("users").child("userID").child("phoneNumber").observeSingleEvent(of: .value, with: {(snap) in

        if snap.exists(){

            //Your user already has a Phone number

        }else{
           //Phone number not available

        }
    })

var userRef : FIRDatabaseQuery = FIRDatabase.database().reference().child("users")
userRef = userRef.queryOrdered(byChild: "phoneNumber").queryEqual(toValue: <NUMBER>)
userRef.observe(FIRDataEventType.value, with: { (snapshot) in
    if snapshot.value != nil {
        //value exists
    }
}, withCancel: { (error) in
    //search error
})