We have a 4-line system in our office, but it is not working properly. For instant, when line 4 rings...it only comes into one of the phones. All lines are working properly with dial tones. If we would answer line 4 on the phone in which it came in and put it on hold... one can not answer that line from a different phone.

Check Forward Busy under Capabilities in programming, any phone that has lines ringing this setting should be set to NONE.
Also make sure line 3 is assigned to ring at those phones.

If unsure of programming the phone system or password call your local tech to come in.

It could be a simple as the phone has call forwarding to another extension set up on it or the direct line could also be programmed on the other set. Does the screen on the non- ringing m7208 display "forward to:xxx" or does it display the current day and time?

they need to be assigned as ringing appearances, what type of system is this?

try to decrease to volpum of ringging
other wise ask the vendpr to reassign the incoming ringging group for yr 3 incoming lines

Factory diagnostic was defective phone.

sounds like your having a couple of problems at the same time. the lines only ringing at station 1 is the default setting of the system my guess is that the time and date are also way off if thats the case.
To change the ringing on a prostar 816 you first need to be at a display set (doesn't matter what the extension number is).
with the handset down enter #20 the screen should say MMC DISABLED
1) enter in 1 2 3 4 1 # (that enables programming)
2) enter in #62 the screen should say day ring assn
3) press the line you wish to program
4) press the buttons for the extensions that you wish to ring (the ones in the right next to the handset)
5) press # when finished with the line
6) repeat steps 2 through 5 for the remainder of the lines you wish to change
press #when finished.

next, the problem with the cordless not working has to do with the port programming, in default it assigns all ports as multiline sets.
To assign a SLT(single line telephone) port to the system while still in program mode dial #38 the screen should say slt dial type and then 16 0's.
0 = keyset 1=slt set
enter in all 16 digits followed by the # i.e.if your cordless is on port 7 it would look like 0000001000000000 #
lastly the phone that can no lnger recieve calls sounds like the do not disturb or DND has been pressed to stop the alarm from ringing. press the mute/dnd button (light should be off). to cancel an alarm press the ALM/SD button twice and then the hold button.
hope that helps out.

pl mention ur model no & make of the system

http://toshibasets.com/uploads/Strata_DK_user_guide.pdf

When you say you are the one responsible for the programming, are you referring to programming the "physical phone" or the PBX and line cards and system functionality?

You should have an extension on your phone. Has that extension for example 222 been translated at the PBX to ring on a direct number -> (xxx) 555-1222

Or by what you are describing, you are more looking at having "shared lines" that ring in multiple locations simultaneously, and once picked up somewhere a "line lock" should be in place?

depends on if any of your phones are set up as programming phones. I do this stuff for our office, you have to make sure the phone is part of the Ring group or ICD group in our setup and that the ring is set to on. We have the software so it helps alot for these little things.