Are you using account codes to make outside calls?
The OS500 does have account codes for access to outside calling if you are using that feature. You can can also call forward off system but every call will take 2 lines and call quality will be poor.
If you dial 9 you will get the first available line out. If you want to call forward a specific line you have to dial it's access code.
Typically but not always; for line 1, dial 701 and line 2, dial 702 and so on.