When customers place orders through Phone, you can update the details by clicking 'Take orders by Phone' option in 'Business' on your dashboard as shown below. Mark whether he is new or existing.
When the customer is new, select 'New customer' option and fill in the details and click 'Add' button as shown below. The details of the customer and his order will be updated.
For existing customers, when you enter their mobile number, their details get automatically displayed and so filling in minimum details such as products ordered and quantity etc is enough. Then click 'Add'. Orders get updated.