@vish You can debug the problem using wireshark according to Marcin.You just try below mentioned code as i already had the same problem and i got the solution
import pexpect
import time,sys
telconn = pexpect.spawn('telnet 192.168.0.105')
time.sleep(20)
telconn.logfile = sys.stdout
telconn.expect(":")
time.sleep(20)
telconn.send("user" + "\r")
telconn.expect(":")
telconn.send("user@123" + "\r")
telconn.send("\r\n")
time.sleep(20)
telconn.expect(">")
I hope this will work.