I know your thinking that all you need is the Net Promoter Score and I will not disagree that it is an important question. However, sometimes just a good old audit and questionnaire might still have some value. You may not be able to do it all in one setting or even ask all the questions the authors of Building Better Teams: 70 Tools and Techniques for Strengthening Performance Within and Across Teams might suggest. The audit is divided into 5 parts with 15 total questions that can be rated 1 to 10 and 2 open-ended questions. My suggestions is to have not only your customers complete it, but your own salespeople and managers. The discrepancy itself might be alarming. 
From the Book:
The process involves sitting down with your customers and identifying opportunities in five key areas:
- Accessibility: How available is your team to your customers?
- Responsiveness: How quickly, and with how much enthusiasm, do your team members respond to customers’ requests and inquiries?
- Listening skills: How much attention does your team pay to customers’ inquiries and concerns?
- Information sharing: How open is your team about sharing information with customers?
- Collaboration and conflict resolution: How effective is your team at being collaborative, especially when resolving problems or conflicts?
The other part of this audit that I liked that it asked how collaborative you were doing conflict. Always a difficult thing to do. Are there other areas they are missing?
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