Two options.
Always route to external or based upon a prompt.
Step 1:
Line management, configure a line that latches an extension. You can even specify office hrs for that. Lets say you use extension 300 for all calls comming on your PSTN line.
Step 2:
Extension 300 Select forward all calls to Outside number. Do not forget to include the dial out number (eg 0) in the number if that does not work, seperate the number by a "," (for example 0,[number to dial]).
Step 2a:
Configure a "Digital Receptionist" Commonly known as the IVR (lets call her BERTA, just to keep it simple

)
The virtual extension must be the same as the one you setup in line management to accept the calle. so that is 300. You can configure a few things here eg have BERTA saying "Press 1 my dear fellow to use the outside line" and than put the divertion number behind option 1 in the config.
To make it even more special what you can do is set up a ring group (real nice stuff). For example what you do is "add ring group" have the ring strategy set as HUNT (works great here, the missus gets a few miles a day now she is running from room to room to get the phone (LOL)). What this will do is allow you to pick up the phone in the case you are at home, if you are not home it goes to the external line. For example 100 is your main extension, 200 second and 300 is (BERTA or direct forward) that can give enough time to answer.
So in short, you can configure:
- by office hrs (In your case when your not home dial always external number)
- by extension direct forward
- by BERTA this gives you multiple options to configure numbers.
I hope this helps.
Henk