First, on a series 1 OBi there is a restriction on the softphone. It must be able to call without registration. There might be one, but don't know of a softphone that will work on an iPhone. You can use CSipSimple for Android or Phonerlite on Windows. The series 2 OBi doesn't have this restriction.
Generally you need a prefix to route to one of the trunks, unless you can identify a unique pattern.
I use a single digit, a 2 followed 10 digits for routing to SP2. Another way is 10 digits to SP1 and 11 to SP2.
Edit:
Linphone is a softphone that will work on an iPhone.