Account Codes
If you have account codes configured on your line then once you have dialled a number which requires a code, you will hear a tone. You will then need to dial a special code before the call is connected.
Your system may use either validated or non-validated account codes. If your system uses validated account codes then you must enter a specific code that has been configured either by your administrator or by you. If your system uses non-validated account codes you can enter any code so long as it is the right length.
If you have account codes your administrator should have explained this to you and told you what codes to use.