When I am in the Job Management Module and I am changing the Customer's details, they are not saving.
There is a setting in Admin / Defaults / Manage Job Defaults, Don't Update Customer's Account Details when saving a Job.
If this is ticked then any changes made will not go back to the Customer Account. It maybe that the person bringing in the item for repair is giving you their phone number, so if the tick is in place that phone number will change the account's phone number.
By right clicking on the outside of the job, the option to Open the Customer appears and any details can be updated there.