There is some confusion about your post. The "Dirt cheap" numbers refer to incoming calls only. As far as I know you would not be able to place an outgoing call on SP2 without signing up for one of their outbound packages. So all of your outbound calls should already have the caller ID that you want, using SP1 on the Obi.
As far incoming calls go, you might be able to ["semi"] permanently have Callcentric forward all calls to your "real" number to your dirt cheap number but they might still charge you the per minute charge for the connection. I'm not sure. To get everything set up right I think you need to drop the dirt cheap number and simply pay the $5.95 per month for unlimited incoming calls on SP1. If you would like to save some money on your monthly charges, you could switch to their new 1000 minute plan for outbound and get everything for $15.90.
*Edited*
Correction on the incoming calls: See this note from Callcentrics web page "Each forwarded call has two legs. An incoming and an outgoing leg. For the incoming leg you would be billed according to the incoming serviceyou receive the calls under."
This means they will definitely bill you for the call to your regular home number, even if it is forwarded to your dirt cheap number.