Problems Dialing Long Distance “I’m sorry the override code you enter is invalid. Good bye” message

hi everybody,

we are having an issue ( trixbox pro v4.1.2 -p13 ) trying to dial us long distance call, some times it work and some times it doesnt. when you press 9 + 1 and the number there is a recording that says “I’m sorry the override code you enter is invalid. Good bye”

i got default rules on dial plan as follow:
9+nxxxxxx
9+ nxxnxxxxxx
9+11
9+411
9+61
9+0
9+n.
9+1nxxnxxxxxx
9+011

i did modify the 9+ nxxnxxxxxx for n+. restart the server and it work. after 2 hours again the “I’m sorry the override code you enter is invalid. Good bye” message.
after one day, works ok and bum! again “I’m sorry the override code you enter is invalid. Good bye”.

could be a problem with the lines ? does anybody had this issue before? will apreciate your comments on any advice how to troubleshoot this.

thanks in advance.