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@Tbs has a PBX code been set up by mistake?
From the user manual
If you're connected to a switchboard, you might need to enter an access code (e.g. 9) before you dial a number. Your BT8600 can store an access code which is automatically dialled before each number. We cannot guarantee that all the BT8600 features will work when connected to a PBX.
Press calls to highlight Base settings, press Select.
Press calls to highlight PBX code, press Select. Enter the number you want (maximum four digits).
If you wish to stop using a PBX access code, go into the PBX code menu and delete all the digits by selecting Clear, then select Save.