For those in the customer service industry, the maxim 'under promise and over deliver' is a well-worn phrase. A common example - offer to deliver a product or service in 7 days, knowing that you can actually deliver it sooner.The intended result - to impress and delight your customer, which in turn should encourage them to buy again and say good things about your brand.

Our experience of consumer insight finds that it isn't quite that simple. For one thing, customers are savvy shoppers - they quickly work out what the 'real' delivery speed is, and start to adjust their expectation of the service, which is usually more relevant to their everyday lives than any small print 'terms and conditions'. Secondly, if the gap between promise and delivery is too wide (from companies getting too greedy about how they want to overperform), it presents an opportunity for competitors to dive in and trump the offer. Why buy an overnight parcel service that guarantees next day delivery by 1pm, when a competitor can offer 9am?

The flip-side of this maxim is of course 'over promise and under-deliver'. No-one will advertise this of course, but a reliable clue for finding it lies in the tell-tale words of "Up to.." Typically, you find these messages not in small print, but in banner headlines, put out by marketing departments driven by sales targets rather than service level standards.

A topical example of this is broadband advertising.

Ofcom, the UK Government telecom regulator, is seeking a change to how internet service providers (ISPs) currently advertise their broadband speeds. Its latest report shows that Britons get an average broadband speed of 6.2 megabits per second (Mbps) – less than half the average advertised speed of 13.8Mb. Internet providers, including well-known brands such as BT, O2 and Sky, currently advertise "up to" broadband speeds, which most customers are unable to receive.

Instead, Ofcom is recommending that broadband speeds should be advertised on a "typical speeds range" – the speed which customers can expect to receive. The regulator says typical speeds must "have at least equal prominence" to headline speeds.

The bottom line here is that ISPs know that signing up to broadband is something customers do infrequently. It usually comes with a 12 month+ contract, and once a customer joins, the hassle and inconvenience of moving again tends to create renewals (not the same as loyalty).

But here's the rub. In days gone by, individual customers might learn from bitter, personal experience that one service provider is worse than another, and may promise themselves to defect to a competitor at the next opportunity. But in today's connected world of the Internet, thousands of customers can quickly tell each other about their experiences, which in turn helps market regulators to identify problems and seek to make changes to protect consumers.

What's the take-out here? Businesses and brands need to make promises to their customers, and if the business is to last, these need to be sustainable. In the long run, neither over or under promising is going to build sustainable growth, either in customer loyalty or financial profits. Instead, we'd suggest some simple, honest guidelines for marketing practice. Far from restricting your brand advertising, it will help focus your business on where you can compete successfully, and build honest respectful relationships with your customers, employees and suppliers:

• Set out in simple terms what the customer should expect – the experience of choosing, buying and using products from your company

•Make it clear how your product/service offer is different from competing brands – this must be believable as well as desirable – good storytelling isn’t enough if it lacks credibility

•Keep the essence of your message simple and honest – only by being truthful and easy for every employee to interpret can it be consistently delivered to customers

What marketers can learn from broadband advertising