I have been selling consulting services for nearly 19 years, and I appreciate very much when law departments treat me with personal and business respect. Law department lawyers and administrators should always regard vendors who are under consideration as fellow humans, deserving of courtesy, explanations, and respect.
It takes but a moment to reply to a phone call or an email. The absolute best action is honesty, an update or two, and even a small dose of empathy can guide this principle: Think how you would feel if you very much wanted to know about what is happening on a decision that is vital to you.
Cold callers and vendors whose services are not sought have less reason to complain about their treatment. My comments mostly concern vendors who are farther into the selling process.
I probably underestimate the degree to which law departments are deluged with vendors, each importuning them. Administrators and the general counsel bear the brunt. Still, it is common decency to honor the golden rule when you deal with vendors.