There is more than one way to do that. One is to make the PhonePort Digit Map prepend **n (where n 1 to 4 depending on which SP you want to use) to certain numbers. The other is to add a little bit to the Physical Interfaces/Phone Port/OutboundCallRoute. Each method has its proponents. I'll use the second one.
The exact characters depend on what you dial to indicate USA numbers and on which SP you want to use.
Assuming you dial US numbers as 001 xxx xxx xxxx and want them to go via SP4, you would add
{(001xxxxxxxxxxx:sp4)},
at the start of the string already there.
If you want to understand how it all works the Admin Use Guide has all the boring details. The relevant bits start at
http://www.obihai.com/OBiDeviceAdminGuide#_Toc367543126Edited. Had forgotten {}.