We typically see about a 10% response to our closed call survey, go here for actual numbers.
If you are seeing lower responses we have a couple of suggestions.
Ensuring that dispatch is getting a valid email is also important, if they are simply picking some default contact that had nothing to do with that service call then the chances of them responding are slim, click here for more. Also if your customer placing the call is a helpdesk ensure they give you their enduser email and not use the same helpdesk email each time.
We have some clients that offer "enter for a free drawing for X" to increase responses and others that will have techs stress the importance of getting feedback to help improve processes (we prefer the latter).
You can also look at the % of times when your customer clicked the link in the email but did not hit submit on the survey. This is usually because there are too many questions, see our suggestion for less questions here.
We also have ID42 - Not Submitting Surveys