Wedding Anniversary Story

John wants to get his beautiful wife, Emma, something nice for their first wedding anniversary.  So he decides to buy her a mobile telephone.  Emma is excited, she loves her phone.  John shows her and explains to her all the different and varied features on the phone.
On Monday Emma goes shopping in the local supermarket. Her phone rings and it's her husband, 'Hi ya, Emma, 'he says, 'how do you like your new phone?' Emma replies, 'I just love it, it's so small and light and your voice is clear as a bell, but there's one feature that I really don't understand though.'
'What's that, Emma?' asks the husband.
'How did you know that I was at Wal-Mart?'