SIP, WhatsApp or a virtual number: choosing lines for AI calls

Before you choose a voice, a model or a script, you have to decide how the agent will make and receive calls at all. The choice shapes what your customers see on their screen, how many calls you can run at once, and what rules apply. This post compares the routes most Indian businesses choose between: SIP trunks, WhatsApp calling and virtual numbers. It is general information as of October 2026, not legal advice, and prices change, so none are quoted.

The routes at a glance

SIP trunks

SIP is the standard way businesses connect phone systems to the telephone network over the internet. A trunk from a carrier or a communications platform provider gives you numbers and a pool of channels; each channel carries one call. Your AI platform connects to the trunk and places or receives calls through it.

Strengths: concurrency you can scale by buying channels, stable business numbers, clean call records. Weaknesses: setup (credentials, IP allow-lists, codecs), and the need for a provider with a properly licensed route in India. Ask any provider directly which route your calls take, which number series they can show on the customer's screen, and how they handle the TRAI rules for promotional versus service calls.

TRAI's July 2026 clarification matters here: it says the 140 series is for promotional calls from entities in all sectors, while the 1600 series is for service and transactional calls from regulated financial entities and government bodies. Which series applies to you depends on the purpose of the call and your sector.

WhatsApp voice calling

Two very different things get called "WhatsApp calling".

Linked personal or business-app accounts

Some tools link an ordinary WhatsApp account and place calls through it. It is cheap to start and customers see a number and name they know. But WhatsApp's terms of service prohibit bulk messaging, auto-messaging and auto-dialling, and bar non-personal use of the service unless it is otherwise authorised. An account running automated calls at scale risks being restricted, and you may lose the number your customers use to reach you. Use this route with caution, for low volume, and with customers who expect your contact.

The WhatsApp Business Calling API from Meta

Meta's Cloud API supports voice calls between a business and WhatsApp users from the same number used for messaging. As of October 2026, Meta's documentation says:

• Calls made by users to the business are free. Calls the business makes are billed per six-second pulse, priced by the destination country, with volume tiers by monthly minutes. A payment method is required.
• Business-initiated calls need the user's permission. Permission can be requested by a message, through a callback after the user calls, or through the business profile. Temporary permission lasts 7 days; permanent permission lasts until the user revokes it.
• Request limits apply: at most 1 permission request in 24 hours and 2 in 7 days. A business can make at most 100 connected calls in 24 hours from a number. After 2 consecutive unanswered calls the user is prompted to reconsider the permission; after 4 it is revoked automatically.
• Business-initiated calling is unavailable for business numbers from the United States, Canada, Egypt, Vietnam and Nigeria. India is not on that list.
• Eligibility includes using the number with Cloud API rather than the WhatsApp Business app, and a daily messaging limit of at least 2,000 unique recipients (test numbers are exempt). SIP signalling is supported as an option, with explicit enablement.

Meta's pages are updated often, and one of them mentions a different per-user figure in a changelog than the main page does, so confirm the current limits on the official documentation before you build a plan around them. This route is the cleanest for compliance, and it reaches only people who use WhatsApp and have said yes to being called.

Virtual numbers and DIDs

A DID (direct inward dialling) number is a phone number you rent from a provider and point at a SIP trunk or an app, rather than a number tied to one physical connection. It is how most businesses get a dedicated inbound line, or a number to show as caller ID.

A virtual number is a label on a route, not a separate route. Its quality depends on the trunk behind it. Ask whether it can both receive and place calls, in which city or series it sits, and whether the provider can register it for the kind of calling you plan. Cora Call also rents Indian numbers in beta, and the same questions apply to any provider.

Comparing them on what matters

Whichever you pick, the telecom rules covered in the rules for automated and AI calls in India still apply, and the line cost is one part of the total in what an AI call costs, line by line.

Questions to ask any provider

Whatever route you lean towards, the same short list of questions separates a line that works from one that causes trouble three weeks into a campaign.

• Which route do my calls take, and is it registered for the kind of calling I plan: service, transactional or promotional?
• What does the customer see on their screen, and can I choose it?
• How many calls can run at once on my account, and what happens to the call that arrives when every channel is busy?
• Which audio codecs are supported? A narrow phone-quality codec limits how clear the agent sounds, and a mismatch can add delay.
• Can I get call records and, where permitted, recordings out of the system?
• What does a failed call cost? Some providers bill for ring time, others only for connected minutes.

A worked example: a dental clinic in Pune

Take a clinic with two front-desk staff that misses calls at lunch and after closing. Its first need is inbound: a number patients already know, answered every time. A DID on a SIP trunk fits, with the clinic's existing number forwarded to it, and a handful of channels is plenty because few calls overlap.

Its second need is reminders the day before an appointment. Patients already message the clinic on WhatsApp, so the WhatsApp Business Calling API with permission collected at booking is a better fit than cold dialling from a trunk. The clinic ends up with two routes for two jobs, and that is normal. Few businesses need just one.

Where to start

Try the same agent on different lines before committing. You can hear one on the demo.

Sources

1. Meta for Developers: WhatsApp Business Calling API, accessed October 2026
2. Meta for Developers: Call permissions, accessed October 2026
3. Meta for Developers: Calling pricing, accessed October 2026
4. WhatsApp: Terms of Service, accessed October 2026
5. SCC Online: TRAI clarifies 1600 and 140 series number framework, accessed October 2026