You should create UIBarButtonItem instead of UIButton:
let backBtn = UIBarButtonItem(image:UIImage(named:?"cellarrow.png"), style: .plain, target: nil, action: nil)
let callBtn = UIBarButtonItem(image:UIImage(named:?"Call.png"), style: .plain, target: nil, action: nil)
self.navigationItem.leftBarButtonItems = [backBtn,callBtn]
The size of bar button should better be 20x20
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