If the mobile number is already registered then in that case you must have registered in the portal already so try to login using the most common passwords that you use for registering on any website . Maybe you have registered and then forgotten . So try doing that .

If this does not work then either write a mail to ICAI regarding this problem or there are some helpline numbers available on the website that you can use to contact icai. You can also visit the nearest icai branch and discuss your problem there.