Silent Call vs Test Call vs A-bis Proxy (ip.access-proxy)

Could someone explain to me the difference between Silent Call, Test Call, and A-bis proxy (ip.access-proxy). To my understanding Test Call opens a pass through UDP port straight to the MS. While SilentCall just creates just creates a open channel. My confusion I believe comes from where you're "sitting" in the eco-system between Silent Call and Test Call. I also am confused after looking at ip.access-proxy. It clearly opens a port for injection either towards the BSC or BTS. However, does a silent-call need to be active to get data to the MS? When I try to inject a simple SMS using ip.access-proxy, (via copying traffic seen across ip.access), I get length disagrees with header. What is ip.access injection expecting in terms of data format? Thanks