•If the forwarding number does not appear in the pick list, select Edit Numbers, and then select New. Enter a forwarding number, including the area code or country code if it is different from your mobile phone number. Select OK.
5.Select OK.
Forwarding calls under certain conditions
You can forward calls to another number when the line is busy, a call is unanswered, your phone is off, or you are outside a coverage area.
1.Open the menus 
.
2.Select Options, and then select Call Preferences.
3.Select the Forward all calls pick list and select Don’t forward calls.
4.Select any of the following pick lists:
•When busy: Forwards calls if the phone is busy.
•Not answered: Forwards calls instead of sending to voicemail.
•Off/No service: Forwards calls when your Treo is not connected to a mobile network.
5.If the forwarding number appears in the pick list, select the number. If the forwarding number does not appear in the pick list, select Edit Numbers then select New, enter a forwarding number, and select OK.
6.Select OK.