The mobile twinning number is used for both mobile and fallback twinning. It is not used for internal twinning. See Twinning.
It should be an external number. If your system users an external dialing prefix you should include that in the number you set.
To do this during a call, press the PHONE key and then press Features.
Use the Bksp to remove the last digit of the existing number.
Use Clear to remove the whole number.
Dial the number you want to use for twinning and press Select.