It's a format that, for better or worse, has migrated to mobile.

Here's the mobile Guardian showing off its latest advert for STA travel.

A Normal Banner Ad? Or Much, Much More?

This being a super-whizzy smartphone, the advert has a surprise up its sleeve. Clicking on it doesn't take you away from the current page - it displays an overlay instead.

Banner Overlay

Brilliant!

This is a fantastic idea!

• You don't leave the referring site - that means you don't have to navigate away from a site you're interested in.
• It's visually exciting.  The advert doesn't contain distracting animation, but it does animate as it unfolds down the site.
• Click-To-Call.  There's a great big stonking call to action "CALL US NOW".  Clicking that will make your phone dial STA's call centre.

So why is it in the Badvertising section?

Let's Click To Call....

Callto?

Ah.

Clicking to call doesn't work.  It's unlikely to work on any mobile phone.

Callto - as far as I can tell - is an unofficial URI scheme supported only by Skype.

There are two common ways to initiate a click-to-call

tel:

According to DeviceAtlas over 2,300 phones support the tel: schema

wtai:

The oldest - and most common - way of dialling from the browser.  The WAP Forum specification was laid down in the late 1990s. The syntax is slightly strange - wtai://wp/mc/ - but it works on many phones.

International

The other problem is that you have no way of knowing where in the world a user's phone is.  You need to specify the international dialling code.  For the UK that's +44.

Why? If the user is roaming and tries to dial 08.... at best they will be unable to connect to your number.  At worst they'll connect to a local number who won't be best pleased to have a bumbling English-person ringing them.

How To Fix It

Using a service like WURFL or DeviceAtlas will allow you to see whether to use tel:, wtai:, callto, or something else.

Use the international dialling code.  You can't assume your customers are based in your country.

Use a geographical number.  Premium rate numbers like 0871 don't come out of your customers' bundles. They can cost much more from a mobile than from a land line.  Make sure the number you choose is suitable.

Test, test and test again.  I can't believe that this mistake cropped up.  Callto: is such a poorly supported URI scheme, that testing on more than a handful of devices would have shown it was unsuitable.

Remember - and it pains me to point out the obvious - if your customers can't contact you; they won't contact you!