SMS, WhatsApp, web chat or voice: which channel for which conversation

Choose the channel by who starts the conversation, how long it will last and what it has to carry. SMS fits a short notice to someone who agreed to receive it. WhatsApp fits a longer exchange the customer opened. Web chat fits a visitor already on your site. Voice fits the urgent case where typing is the obstacle.

Most teams choose the channel by habit, and then ask the agent to cope. It is cheaper to choose by the conversation.

Does who starts the conversation decide the channel?

More than anything else. A customer who writes first gives you permission to answer on that channel. A company that writes first needs consent, and every channel has its own version of the rule.

On WhatsApp the rule has two parts. A customer message opens a 24-hour window in which you can reply freely, and outside it only templates are allowed. Separately, Meta says you can only message people who have opted in:

you can only send messages to WhatsApp users who have opted in to receiving messages from you.

SMS has no window, but outbound marketing needs consent in each of the four places we look at. We list the rules for Australia, the US, the UK and the EU in a separate post.

What does each channel carry, and what rule bites?

Why does length matter so much on SMS?

Because the limit is smaller than it looks, and one character can halve it. A standard SMS holds 160 characters only when every character belongs to the GSM-7 alphabet, as set out in the 3GPP technical specification for SMS. Add one character outside that alphabet, such as an emoji, a curly quote or most accented letters, and the message switches to UCS-2 encoding, which holds 70. Longer messages are split into segments of 153 or 67 characters, and each segment is billed as a message.

That matters if you write in Spanish or Portuguese, where accented letters are everywhere, and it matters for an AI agent in particular. An agent that writes freely will happily produce a 400-character answer with three accents. On SMS the answer has to be written to fit, and the cost of the long version shows up on the invoice.

Which channel for which conversation?

Five common cases, and the channel we would start with.

These are starting points. The right answer for your business depends on where your customers already are, and you can see it in the channel they use when they contact you first.

What happens when the conversation changes channel?

This is where an agent earns its place. A person who starts by SMS and then needs a call should not have to repeat the problem. The agent has to recognise them, carry what was said and tell the next channel what is left to do.

That is a question about identity and memory, not about channels, and we cover it in the post on what orchestration means. The handoff to a person raises the same question one step further, and we cover that in its own post.

What does an AI agent need differently on each channel?

The same intent needs a different shape. On SMS, one short sentence and a link. On WhatsApp, a few lines, perhaps a button. On web chat, the page can show the form itself. On voice, one idea at a time and a confirmation of anything that matters. Voice has its own post, because the rhythm of speech changes more than the format does.

An agent built for one channel and pushed onto another keeps the wrong shape. Writing the answer once and adapting it per channel is the part that is worth building.