Set up a Callcentric account on SP2. For these purposes, a free account will be fine.
My own OBi is far away and I don't want to risk losing control of it by testing some digit map changes, so I'll leave those up to RonR. However, I'm fairly confident of speed dial setups, so I'll post those without testing them.
If you set e.g. Speed Dial 2 to SP2(**275*18005551212), then dialing 2# should place the call via Callcentric's "free" route.
Set Speed Dial 3 to SP2(18005551212@sip.tollfreegateway.com), then dialing 3# should place the call via Tollfreegateway.
You can compare these for setup time and voice quality. There are also several other free toll-free termination providers.
Once you are satisfied, you can modify the digit map to route 800/888/877/866/855 automatically.
Early Media refers to audio played before a call is "answered". Common examples are announcements that a number is not in service or has been changed, that a cellular customer is out of range, etc. The idea behind "not answering" is that the caller is not charged minutes for hearing the message.
In the case of toll-free (where it's the callee that's paying), big users have a lot of clout with the carriers and can negotiate contracts, where they don't have to pay minutes until the call is transferred to a human. In the old (pre-1990) days, the only way to delay the start of billing was to not "answer" the call. Although there have been better solutions for 22 years, some of these clowns are still doing that. Bell-shaped heads often move very slowly. Combined with the turkeys at Google who don't seem to realize that Early Media is important, the result is systems that don't work properly! Sorry for the rant.