My purpose is to do web service calls from Amazon EC2 instance to external provider. The web service provider only allows calls from predefined IP addresses due to security reasons. Is getting an elastic IP from Amazon the solution here, or does it only work for calls TO my EC2 and not FROM EC2?
Does having multiple EC2 instances behind ELB (for High Availability) add anything to this?
Yes, getting an elastic IP address would solve this problem.
If you have multiple instances behind a loadbalancer you don't get a static IP address for the loadbalacner and so you would need an elastic IP for each instance.