Luxoria - 4:29 am on Oct 30, 2011 (gmt 0)
When I get a get a failed cart or payment I investigate the problem to make sure the problem is not on my end then I send out a friendly email reminder. (or a solution if there was a problem)
I absolutely will never call a customer. That goes beyond a considerate response. Very few people do turn around and complete an order after I contact them, mostly those people are the one's with technical issues. (such as wrong state for the zip code, or they forgot their PayPal password and so forth)
My volume is low so I have the time to do this manually.