Inbound SIP URI calling for Voice API (EA)

This expands our BYO-AI capabilities by making it easier for customers to integrate other applications, including voice agents, directly with the Voice API. By enabling inbound calls to be received via a SIP URI (rather than relying solely on traditional phone numbers) and supporting SIP REFER for transferring active calls, customers gain more flexible call routing options and a simpler path to connecting the voice infrastructure of their choice.