Sonic Fusion "20/1" is just the max, can't be guaranteed. Looks like it's just a matter of not receiving correct information about the speed when you signed up.
They don't limit your speed, but since you are so far away, your speed is limited via natural causes, e.g. line distance. They should probably make this point more clear in their sign-up materials.
I doubt this has much to do with AT&T lines. They always provide service through AT&T lines, don't they? Also, I don't think they have VoIP with Fusion, I think they're supposed to give you PSTN right on your copper wires, no?