Towards the end of the sales cycle; prospect became very quiet, and does not return most of your calls. what does that tell you?

I never get dissapointed till its finally over. In this case rather than calling the prospect again and again I will try to get an appointment with the prospect . The purpose of meeting should not be on focussing on closing the deal it will make the prospect irritated but it should focus on the future course of action by getting insights how our product and services will help him in meeting his needs.

Looks like you have not done your work at the customer place properly 

You have not answered his quiry , may you have not given him the option to come out with his quiery  or have not prospected him properly [In the SPANCO process ]

The prospect is simply not interested in talking to the respected sales rep anymore and calling again and again would further drive the prospect away , However in this situation a call from a senior sales person or some one with a higher designation for eg a manager could prove fruitful as prospect would most likely give the person more respect as compared to the sales agent whose incompetency drove him away.

Consistently calling is very annoying, I would set up an appointment and hear it from the horses mouth itself. There is also a possibility that the prospect needs further pampering one on one and that needs to be done in person. Not answering calls does not mean that one has lost the prospect.