A SIP address is for SIP/VOIP communications. It looks a lot like an email address, and ideal for allowing compatible carriers to call you without it costing anything. Therefore, someone in India with a SIP provider will be able to call you for free just by using your SIP address. For PCS1, a SIP address will be the full telephone number followed by @pcs1.ws. For example, if +1 412 555 5555 was a PCS 1 telephone number, then SIP address will look like sip:14125555555@pcs1.ws. The only thing the caller needs to do is dial the SIP address in their VOIP client. This will also work with cellular lines – therefore giving flexibility no matter where the customer is.