Networking problem HP IPAQ 6828

After installing the sim card in my phone when I switch on it,it keeps on searching for availablenetworkand when I switch ittomannualsearch,itisabletosearch the networs awailable butwhen I go on for selecting the desired network ofmy service provider, a message appears"unable to registered,please disconnect the data cells"

Please let meknow why this is happening and what is the solution

R.K.Maini