You are going to have to go into your ESPT settings and probably change some things. I just had this problem...maybe not even an hour ago and it was a long process but i got it fixed.
You have to dial ##778 on your phone and press send. When it has the 3 selections, you must hit "Edit". Now you have an unlock code you must enter. Normally it is 000000 for an Alltel phone (but my phone was unlocked for either Sprint or Alltel so i had a special one...got it off ebay..
). If you can get in, you have to change your settings (i believe your M.IP stuff). I can elaborate a little more later but i have to leave for a meeting, srry. Ill be back on later today.