gworona,
I have a Sipgate account and have verified in the past that it works fine with the OBi. If necessary, I can try it again (just not at this moment).
QBZappy's suggestion is an excellent one and would verify that you're using the correct username/password.
If it turns out you can successfully register with Xlite but not the OBi, I will put my Sipgate account on the OBi and also try your credentials if need be.