Check if u have installed microfilter at every telephone socket
except the one which yur modem is connected to? If yes, check if
one of yur microfilter is faulty.. to check, make sure u online
liao, use tat particular telephone socket u wanna check, pick up
the phone and dial to someone.. if immediately disconnected, tat
means most probably tat the microfilter attached to tat telephone
socket is faulty.. if doing all these still no prob, maybe yur
modem might be faulty...
But if after doing all these method, still got prob, u must call
singnet liao.. maybe yur line is poor or other probs.. hope all
these info can help u.. 
