HELP! Home Telephone - No Dial Tone - Have Dial Tone at Outside NID - What else can I do?

• I have POTS (Plain Old Telephone Service) home telephone and DSL internet.  (I do not have any U-verse services.  Not available in my area yet.)
• For several weeks, there was some static on my phone line and then a few days ago, I lost home phone service altogether.  I have no dial tone on any phone.  My DSL internet service continues to work.
• For a few random times, the home phone service could be restored by calling my home number from my cell phone.  It was a temporary fix but I would have a dial tone again for a couple hours.  That process only worked a few times and is no longer working.
• One of my cordless phones shows "Line in Use" on its display.
• I have a corded telephone (good old cheapie $9.99 plug-in handset/test phone) that I can use to test jacks.
• I tested the line at the NID box outside my home using the cheapie plug-in handset.  I do have a clear dial tone at the NID and can make outgoing calls from the NID.  So if I understand everything I have read, that indicates the problem is either with the inside wiring, inside telephone equipment, inside DSL filters, or somewhere else inside the house.
• After the home phone service was lost, I got an error message from my Netgear modem that it had detected problems with my DSL service.  See attached print screen.  I ran "Check Connection" tests on the modem and received "Fail" for IP and DNS tests.  Looking at the "Details" of the IP error, the failure was in the "Continuity check to 6rd BR".  The "Details" of the DNS failure show a failed query and ping.
• I was hoping it would be a bad DSL filter - even the modem's warning indicated that.  I removed every phone and every DSL filter from every phone jack in the house.  I turned the computer and modem off and unplugged the modem from the wall jack.  But using the cheapie phone, without a DSL filter, I still cannot get a dial tone in any jack.  So I do not think the problem is with my phone equipment, DSL filters or modem.  If I cannot get a dial tone using a known working phone plugged direclty into any jack, that eliminates failure of the equipment, DSL filters, and modem, right?
• What else can I test? What else can I do to troubleshoot and fix this myself?
• Is it possible that the problem is in the wiring inside the NID box itself?   In my reading on the internet, I am led to believe the problem could be there.  If so, whose responsibility is that to pay for repair of wires in the box:  AT&T or the homeowner?
• Any other ideas what I can do to fix this before calling an electrician?  I do not have the Inside Wiring Maintenance (insurance) coverage/plan.
• For several days, the phones have produced no sound at all (completely "dead" and silent).  But after fussing with the jacks, filters, etc. while writing this post, I am now getting a loud, constant "full" static sound.  Hmm?  Only the computer modem and one phone are plugged in (different jacks). 

Thank you very much for reading this and any help you can provide.  It is greatly appreciated!  (Attached photos are below.)

Carol