He was going to give up Cheyenne to be with her, one of the biggest sacrifices a cowboy could make until she said she was leaving him for another. The song describes the cowboy ending the call and leaving the phone off the hook and slowly turning around and taking one last look. He then gets in his truck and "aims it toward that Wyoming line."
He was prepared to surrender Cheyenne in favor of her, a monumental sacrifice for a cowboy until she revealed she was abandoning him for someone else. The song depicts the cowboy ending the call, leaving the phone off the hook, and then slowly turning around to take one last look before getting into his truck and driving towards the Wyoming border.