We digged the problem, traced sqlplus command and find out some socket issues.
socket(2, 2, 0) = 12
_esendto(12, 0x0FFFFFFFFFFF60A0, 26, 0, 0x09001000A0025530, 16, 0x0000000000000000) = 26
_poll(0x0FFFFFFFFFFF5260, 1, 5000) (sleeping...)
_poll(0x0FFFFFFFFFFF5260, 1, 5000) = 0
close(12)
After talking with network team, they found problem on dns server and
changed dns server address, issue is resolved.
OS: AIX 6.1
DB: 11.2.0.3
You can trace sqlplus with below command;
truss -o truss_sqlplus.out sqlplus / as sysdba