orangelau,
I know you have an OBi110, so let's assume that you have the default:
Physical Interfaces > PHONE Port > PrimaryLine: PSTN Line
To do what you want add a rule like so:
Physical Interfaces > LINE Port > DigitMap:
(<**1777>1122334455| ... existing rules here ... )
If you wanted to make it a more general rule to add 777 to all ten digit numbers starting with 1122 and route them via sp1, then a rule such as this would work:
Physical Interfaces > LINE Port > DigitMap:
(<**1777>1122xxxxxx| ... existing rules here ... )
For both cases you need to add a rule that allows sp1 to accept a thirteen digit number starting with 777. Like so:
Service Providers > ITSP Profile A > General > DigitMap:
(777xxxxxxxxxx| ... existing rules here ... )