How about I go and wait until our receptionist is on the phone to leave the office?
It sounds like you need to be a ninja. How will you be able to leave the office unnoticed?
I'll walk out while she's busy talking with someone on the phone.
When you are an employee, it's expected that you will be at work during regular office hours. It is wrong to try to sneak out early as it leaves the work that you are supposed to do to others in the office. You shouldn't try to sneak out of work and instead leave when you are supposed to or otherwise you risk getting in trouble and possibly fired.
So I have to spend my time at work doing nothing while other people are talking to each other?
Unfortunately, yes, you need to stay at work and do nothing rather than sneak out. It's important to follow work procedures and that means you need to be at the job location. At least you are just chilling and waiting out the clock and not doing anything strenuous.