Cisco IP phone 7841 8851 unable to forward all to external number

[badwolf1686]

Hi, we are using Cisco 7841 and 8851 IP phones on Unify OSB system. The phones are registered through TFTP server, and internal/external calls are fine. For 'forward all' functionality, the user can only dial four digits and it just calls out. I guess the defaults are forward all to an extension. Is there a way to forward all calls to a cell phone (0+11digits in my case)?

The loaded firmware is sip78xx.14-2-1-0201-40 and sip88xx.14-2-1-0001-14. I'm aware that it's not 3PCC version but since the softkey is working, I assume it should work with some tweaks.

Thanks in advance!