I acquired a ProStar STC 816 key service unit, problem being I can't find any online user guides/manuals! Could you give me a web address, please?? Thanks, in advance! (Model: PROSTAR 816 KSU)

Hi
I found a manual for your phone here;
http://pdf.textfiles.com/manuals/TELECOM-S-Z/Samsung%20Prostar%20816%20Plus%20System%20Admin%20-%20Specia.%20Features%20Guide.pdf
and it would appear (page 2) that your phone / zone is operating in "Night Service Operation" which presumably mutes the ringing tone.
You can download this file (pdf format) to your computer for future reference.
Hope this is helpful, if so would you please register that with Fixya
Cheers

Try to turn on and turn of the unit then check for the settings if theres available if theres no settings for it check the key pad of your phone you can see a diagram like volume just make a long press on it

Hi, you can set Do Not Disturb on the prostar 816 by dialling 401, to take off Do Not Disturb dial 400, leave the handset down when doing this, a softkey can also be programmed as a DND key if this makes life easier. I have uploaded a video for you to look at, there are lots more on YouTube search abbeytel, or visit www.abbeytelecom.com. Hope this helps.

Once you start using the Conference key on the Prostar 816 you will find it really easy to use, and very helpful! I have uploaded a training video for you to watch, other prostar 816 clips are available on Youtube searching for abbeytel. Visit www.abbeytelcom.com for written copies.

It's not called a Prostar in the UK but is known as the Samsung DCS 816. The follow clip will show you how to add to the system speed dial directory. But you can find 60+ handy clips at www.AbbeyTelecom.com

There are written user tips on http://www.abbeytelecom.com however I have posted 1 of 68 training videos for the Samsung Prostar which are more current than the 816 but easily adapted to the older handsets, the codes are the same.

Possibly, does the system shut the line down or does it just doesn't break dial tone?

Transfering on a Prostar 816 is done by using the DSS buttons on the right hand side of the phone. This is accomplished by having your outside call in progress and pressing the DSS button of the persons extension you wish to transfer it to. If you want to screen the call wait for the extension to answer otherwise hang up and the call will be transfered. Transfer keys were only placed on the 800 series sets and was a non movible button.
hope that helps

The Prostar 816 phone system can only use Prostar 816 phones. There is a rare larger Prostar system that can use other Prostar phones along with the 816 but typically was only installed in situations where more than 16 phones were needed. If you are looking at non-prostar phones like the Samsung DCS 12 then they certainly will not work.