One of the requirements of the ISO9001 standard is measuring customer satisfaction. How you measure this is up to you. The standard states that Monitoring customer perception can include obtaining input from sources such as customer satisfaction surveys, customer data on delivered product quality, user opinion surveys, lost business analysis, compliments, warranty claims and
In my opinion the best way involves a combination of methods. The simplest method for a company in a service industry being developing a basic survey to attach with your invoices.
The whole point of implementing a QMS is to improve quality, profit and efficiency so the perception of your customer is massively important. Try not to take any negative feedback personally (although this can be difficult) and use it to improve your systems.