I left the job last week can I phone them to apply again?

I don't know, can you operate a phone? 

Yes you can, and your former supervisor will tell your former coworkers that you did, and everyone will laugh at you.

Depends on why and how you left. 

If you didn't like the job, or you abandoned it then don't bother going back to it. 

If you left on good terms then you can try and apply. Call them to see if they're hiring don't call them to apply for a job. You can do that on line. 

Maybe, if they valued your services, but you need to explain why you left.

Depends on how you left. Some companies are willing to let you try again while others don’t. Just give it a shot and if it doesn’t work out there’s always plenty more jobs out there. Best of luck!