I think the key thing is for there to be a
reason behind the call (other than "we
hope this will help conversions).
If you're early stage and doing customer development it makes sense because their input actually will be important to the direction of the product. If your product is complex, comes with a range of options and/or is very niche and high-value it makes sense because users feel your call actually might help them. If your ideal client is the VP Business Development or Marketing Manager and effective use of your product directly affects their bottom line they probably want to talk with you, and if you're selling a solution to a particular large enterprise problem there's probably someone whose job is talking to people like you.
On the other hand, if your typical customer is a mid-level software developer who can probably grok what your product does on the rare occasions they're likely to use it, they'll call you. And there's something quite dubious about most consumer products sold through cold calls.
Less intuitively, senior people within organizations (if you can reach them) are often much more willing to discuss in detail because they're used to being asked for feedback and seeing that feedback implemented, and making purchasing decisions isn't a painful process for them. Their career advancement has often been aided by their willingness to listen to people with useful suggestions too...