Dead phone jack

Cox came out and removed outside phone box and put a phone modem in my guest room (not connected to my computer which is in another room).  It is just plugged in and connected to existing phone jack.  About two weeks after it was installed the phone jack in my den where my base phone is connected went dead and all cordless phones said "check phone line".  I found an old corded phone and plugged it into the tel. 1 line on modem and it had a dial tone. I then brought the base phone to the modem and plugged it into the modem and all 4 of my cordless phones had dial tones so Cox said it had to be my problem with the phone jack in my den. What I don't understand is why a 3 1/2 year old phone jack would go bad (it was brand new when we did a remodel) coincidentally that soon after the modem being installed. It worked just fine before that. I have left the base phone plugged into the modem so I can have phone service but my problem is two of the phones are very staticky and will drop calls when you are talking on them. I assume it is because they are two far away from the base.  That really shouldn't be the problem because before the modem was installed I could use a handset in the back yard or next door at my neighbors. Before I start putting in another new phone jack can you please advise if there could be a reason for the phone jack going dead and the modem being installed? I would truly appreciate any help you can provide. Thank you in advance.