You can use the key fob to operate Summon, avoiding the problem of your mobile phone not having a signal.
To do that you need to disable "Require Continuous Press" in Settings. Which of course removes a level of control you have over the car. So be aware of that limitation.
At
https://www.tesla.com/sites/default/files/Model_S_release_notes_7_1_1_us_cn.pdf
QUOTE:
"Use the key fob to Autopark or retrieve Model S: Press and hold the center button on the key fob until the hazard lights flash continuously. While the hazard lights are flashing, press the frunk button once on the key fob to drive Model S forward into the parking space or the trunk button once on the key fob to back Model S into the parking space. Model S will move up to 39 feet or until the sensors detect an obstacle, at which point parking is considered completed and the car will shift to Park."