mobile phone number hyperlink

Hi

I built a site and wanted to have a click to call button on the mobile version, but when I put the mobile number in, and tap on a mobile phone (when the site is published) it doesn't work.  I am in the uk and wonder if its the format I used.  I just put in a normal uk number eg:  09786 453658 (not a real number, just the format I used).  Is this where I have gone wrong?  Hopefully someone can enlighten me! Thank you

Michele