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SIP trunk vs virtual number for a voice agent

The two things a voice agent needs to be on the phone in India, a number and a way to carry calls on it, why they are sold separately by some providers and together by RingTrunk, and what to check before choosing.

Updated 6 September 2026

In short: a virtual number is the address people dial; a SIP trunk is the road the calls travel on between the phone network and your software. A voice agent needs both, and RingTrunk sells them together: an Indian number attached to a SIP trunk you point your own platform at.

Definitions

  • Virtual number. A real Indian phone number that terminates in software rather than a handset. On its own it does nothing; something has to receive the calls.
  • SIP trunk. A connection over the SIP protocol between your voice software and the telephone network. Your platform authenticates to it to place calls and accepts its INVITEs to receive them. On its own it has no number to present or be reached on.

Why they come apart in some products

Many number products bundle a hosted phone system: a web dialer, an IVR, recordings, an app. Convenient for an office, wrong for a voice agent, because the agent already has its own audio pipeline and needs the raw call, not a phone system in front of it. Some trunk products, on the other hand, assume you bring numbers from elsewhere.

What a voice agent actually needs

  1. A number people can dial, that also serves as caller ID.
  2. A trunk that delivers incoming calls straight to the agent's SIP bridge and accepts outgoing calls from it, with no menu or app in the middle.
  3. Rules enforced for you: Indian caller-ID requirements, domestic destinations, capacity that does not touch calls in progress.
  4. An API, so provisioning a number for a new customer is a request, not a ticket.

What to check before choosing

  • Does the number arrive as plain SIP, or only through the provider's own app or SDK?
  • Can inbound calls go to your server by source IP, with failover across more than one address?
  • Is the caller-ID rule enforced before the network rejects the call, so you get a readable 403 instead of silence?
  • Is the codec stated? Indian mobile networks carry G.711 A-law; a platform that only offers Opus will not connect.
  • Can you script all of it?

On RingTrunk the answers are: plain SIP at pstn.ringtrunk.com, inbound by source IP with ordered failover, 403 cli_not_allowed enforced per call, G.711 A-law, and a REST API for everything. Start with the documentation overview.