If you have a landline (PSTN) connected to the Line port of your OBi, you can use it as a trunk on Asterisk to make calls, without the auto-attendant. See http://www.freepbx.org/support/documentation/howtos/howto-use-an-obi-110-device-to-provide-to-allow-freepbx-to-make-calls-o
(ignore the part about Google Voice).
Incoming calls could be sent to Asterisk's DISA if you like, but you would want to prompt for a password before giving dial tone; otherwise anyone calling your number could make expensive calls through your Asterisk. If you want a delay before sending the call to Asterisk, add it to the InboundCallRoute for the Line port; see the section 1.3.0(2575) in http://www.obitalk.com/forum/index.php?topic=9.0
Sorry, but I don't completely understand your question. What service do you currently have on SP1? If this is a Guatemala SIP service, why don't you use it with Asterisk directly? Does it have a DID, separate from the number on your landline?
Are you using FreePBX or other GUI with Asterisk? If so, which one?