How to completely delete the deleted SMS from Apple's mobile phone?

At present, only one contact can be deleted at a time in SMS (swipe from left to right or swipe from right to left to select Delete), and all SMS messages cannot be deleted. If you need to delete all conversations of a contact, please enter the contact, select Edit, and then select Clear All. To delete only conversations, select Edit, click the conversation you want to delete, and then select Delete.