Device status vs presence status

Hi fellow forum members, I have been using Asterisk for years and used a considerable number of handset types as well. My first experiences were with Cisco 7940/60s using SCCP via chan_sccp but I soon moved to the SIP load. Next I discovered Polycom IP600/650 handsets and since then have played with Yealink, Digium, Snom, & Grandstream. Generally I found that users really find it useful to see the status of commonly used extensions via integrated BLF keys or side-cars that provide this. The days of simply dialling an extension on the off chance that it is not in use are perhaps coming to an end - users just expect more. Here comes the rub though, why does the average BLF not actually show that a device is not registered at all? I have heard so much conflicting information on this; some manufacturers claim that this is part of presence and that their handsets won’t subscribe to presence! To my mind, the fact that a device is not registered is a device status and nothing to do with presence. I really don’t want presence (users indicating on their handset whether they are available etc). Not since the Polycom IP600/650s have BLF shown an extension is not available at all and these handsets did indeed support presence. I find the documentation on the differences between device status and presence status totally confusing and so do handset manufacturers I suspect and opt just not to bother understanding it. Can anyone recommend me to a handset with BLFs (with LCD display of current usage) which will show when an extension is not available at all? Thank you so much.