Holly texting a landline would have replied with an 'undeliverable' error, so she would have known that he had never gotten the message.
Austin says that a timber hitch or sheep shank should have been used to tie Holly's tree on her car. A timber hitch is only useful while under tension and is normally used for towing a cylindrical object. A sheep shank is also only useful while under tension and is used to temporarily shorten a rope. Neither would be used to tie a tree to a car roof.