Call forwarding icon automatically appears after power on though call forwarding off

I'm using iPhone 5 with iOS 6.1.3. and I have this issue: the call forwarding icon automatically appears after I turn on the phone even though I haven't turned on the call forwarding function! I can make the icon disappear but when I power off the phone and turn it back on, the icon appears again. Since in fact I haven't turned on call forwarding, I can still ring my iPhone despite the icon appearing on the status bar, which proves the call forwarding icon is simply a dummy one in this case.

I tried to make the call forwarding icon disappear by using the method stated in this Apple Support article:
'Check Call Forwarding status by tapping Settings > Phone, and then tap the Call Forwarding option.'
(http://support.apple.com/kb/TS2795)
And by dialling ##21# using the phone app to deactivate 'All Call Forward'
(http://www.three.com.hk/WebCS/Educat...d=WCSShortcode)

I don't know if the following instructions can be used worldwide but at least on 3 Hong Kong network:

*#67# -> check the status of 'Busy Call Forward'
##67# -> deactivate 'Busy Call Forward'

*#61# -> check the status of 'No Answer Call Forward'
##61# -> deactivate 'No Answer Call Forward'

*#62# -> check the status of 'Unreachable Call Forward'
##62# -> deactivate 'Unreachable Call Forward'

*#21# -> check the status of 'All Call Forward'
##21# -> deactivate 'All Call Forward'

After several trial and error procedures, I find that the Settings > Phone > Call Forwarding option basically functions as the **21* + phone number process (activate 'All Call Forward') and the ##21# process (deactivate 'All Call Forward').

==========

If you search for 'ios 6 call forwarding icon' on Google, you can see there are lots of people having this problem around the world.

The reason why I encouter this problem maybe because I entered the code *#67# to check the call forwarding status just out of interest. And now I have this issue...