Your system has lost its programming. #104 showing that there are 8 lines means that all phones have all 8 lines assigned to the on the first 8 buttons. A line that is programmed as a line button can not be programmed as anything else, until you remove that line from that extension.
I think it's too screwed up for you to ever find the incorrect programming that has gone on, and you should default it and start over.
Please find the programming overlay for the phone, knowing the location of Next Procedure, Next Item, and Next Data is going to make your life much easier.
Once the system is defaulted, go into System Programming. Press Next Procedure, which will take you to the date. Once you set the date, press Next Procedure again, which till take you to the time. Set that, Next procedure, you're at number of outside lines. I think you're saying that you have 6 outside lines, so dial 06.
Continue using Next Procedure to step through all of the programs. When you get to one that is specific to an extension, such as Automatic Extension Privacy, press Next Item to select Ext. 10, and set it as required. DON'T set all of the extensions, just set Ext. 10. Continue with the Next Procedure until you end up at the date again.
Now, dial #399 (copy), 10 (the extension to copy from), and start dialing 11, 12, 13, 14, 15, 16, 17, etc until you get to the end of your extension range. This doesn't program buttons, but it copies everything that you set 10 to do to the other phones. If you have phones that don't ring, you address them one by one in Centralized Extension Programming.
Finally, if you have Voicemail, you need to know the extension ports of the Voicemail, and use #505-7 to assign them, or incoming calls will not work until you do so.
At this point, only buttons 1 through 6 will have anything on them, all of the other buttons are blank, and all of the erroneous programming that is keeping you from being successful will be gone. You can program your CID buttons, your Auto Intercom buttons, Redial, DND, etc. as needed without issue.
If you don't have a manual to reference, you can get a pdf
here