RULES Customer-perceived rule: If you can't reach the sales office (Carbonite) you can't purchase the service.
Enterprise rule: If inbound sales exceeds n calls / hour, push all calls to voice mail.
From an enterprise's perspective -- the seller in this instance -- the sales office's inaccessibility may be unintentionally salutary. It limits customer volume at a time when the enterprise's infrastructure and staff may be unable to handle the volume. Of course, sales are lost, and customers must question why the sales staff is unavailable during peak daytime periods, but the enterprise rule could serve its purpose, at least as a stopgap measure during an unusual circumstance.
Later facts that came to light about Carbonite (a hosted provider of backup services) suggest that this rule may not be de facto in their case, but nevertheless a capacity-based trigger such as inbound phone sales inquiry volume remains an illustrative use case.