There's no way to implement a 5-minute maximum per call rule. Think about it.

What this almost certainly is instead is a badly communicated 5 minute average per call target, which (vary duration by company/phone line) is necessary in any call center as it's one of the only ways you can manage costs, forecast correctly and get your speed-of-answer right.

I say badly communicated as the staff should realize it's not an absolute per call and should also not be communicating as such with customers. The 5 minutes (or whatever) should be averaged out over a longer period of time (e.g. a week, or even a month) and should be arrived at through ensuring good employee knowledge, efficient and reliable systems for them to use and general all-round coaching, training and support.