Channel is not created

Hi friends,
while i am trying to communicate with two SJphones(Clients), the server (Asterisk) is not able to create the channel, in the log file it showing the following error messages.

391 [Oct 6 17:31:04] WARNING[6074] app_dial.c: Unable to create channel of type ‘SIP’ (cause 20 - Unknown)
392 [Oct 6 17:32:37] WARNING[6079] app_dial.c: Unable to create channel of type ‘SIP’ (cause 20 - Unknown)

Can any one tell why the channel is not created?