I have 4 SIP accounts defined in my OBI200. I can accept incoming calls on all of them, but I have problem with outgoing calls on line#3 only: when I dial **3<phone number> I get error saying "The number you dialed **3<phone number> was rejected by service provider. The reason is 400".
I can dial from all other lines (default one, **2, **4). I changed provider on line#3, but the problem still exists". It looks like Obi200 does not accept **3
Try and change your primary line to SP3 and see if calls go out ok. If not, there may be a configuration problem with SP3.