ok, i got it. you only have phoneline connected. have you tried unplugging the phoneline from the back of the receiver and hooking a regular phone into it to test for dial tone and clarity? make sure there is no issue with that particular jack and cord?
also, if you have dsl services, there might be too much "dsl noise" in the phoneline for the modem to function correctly. perhaps a dsl filter is needed or if you already have one, try changing it out. they can fail over time.
next item to check is do you have a voip phone system? some broadband phone carriers do not support analog modems over their system. i believe vonage can if you program your receiver to dial *99 first. (this can be done at menu 6-1-4 in most receivers). perhaps recently your provider no longer provides analog modem coverage over their system and they previously did. or did you recently have a receiver replaced and its not working since then? maybe the prefix your carrier requires has not been entered.
and of course, it could always simply be the modem in the receiver itself, or the problem could be completely on dishes side of the phone line. maybe their servers which support the receivers are not working correctly. look how many times the dishnetork website goes down.
hope you find your resolution soon. keep us posted.