Dan231,
If they want all calls to go to the main call center from this branch then you will need to go to.
System > Devices and Feature Codes > CO Trunk Groups
Let us know what is in that section. Hopefully there is only 1 CO Trunk Group 92001 or 9201 depending on how old your software is. Of course the initial programmer may have changed the numbering plan and it may be something else, but just let us know how many you have in there.
Now, once you have looked at how many there are then open them up one and a time, if there are more than one, and look at the follow two lines.
Day Ring-in Type
Night Ring-in Type
Those two values tell the system what the trunks associated with that trunk group will route to when a call comes in on those lines.
You will then need to look into the main site's system and see where you want the calls to go to, a Hunt Group, a STAR, a Phantom Routing Extension, etc..
Then we will be able to help you design the routing for that site.
Thanks,
TE