I'd like to work on a SIP project for mobile devices. I've seen links to Siphon, Sipdroid and OpenSIPS.
Does OpenSIPS allow for me to make phone calls between two iOS devices in a local network?
In other words, I'm trying to do an app for a VOIP company. I'd like to set up my own SIP server and then use theirs later. Is there any reason that it would not work?
OpenSIPS is a SIP server application that includes a variety of components that are often needed for a SIP infrastructure such as Registrar, Proxy, Presence Agent etc. What OpenSIPS is not is a SIP user agent (or at least not one that's designed to work with a person as the end user). That means it's not built to make or receive calls.
If you are planning on using OpenSIPS as the SIP Proxy on your network combined with a different piece of software, such as Sipdroid, acting as the user agent on your mobile devices then the answer to your question is yes. You will be able to configure OpenSIPS to allow calls between devices on the same local network.