Channels are not interchangeable: the most common early mistake

It is tempting to treat channels as interchangeable pipes: write send(to, text) once and pick SMS, WhatsApp or email at the last moment. That kind of shortcut is useful. It is also the source of most early trouble, because it invites you to believe the channels behind it are the same thing with different logos.

They are not. They differ on four things that matter.

• SMS: a business may start the conversation, provided it has registered its sender where the country requires it and holds the person's consent.
• WhatsApp: a business may only start with a template — a message format Meta has approved in advance. A free-form message to someone who has not written to you in the last 24 hours is refused. (See the 24-hour window.)
• RCS (the richer successor to SMS built into phones' default messaging apps): broadly the same rules as SMS, plus brand verification, and only where operators support it.
• Email: anyone can start; abuse is punished later and quietly by spam filters.

A system that models "send a message" without modelling "am I allowed to start this conversation?" works in testing and fails in production.

• SMS is plain text in segments. One segment holds 160 characters in the basic alphabet, but only 70 if the message contains characters outside it — Arabic, Hindi, Chinese, Greek, or even a single emoji. Longer messages are split and billed per segment.
• WhatsApp templates must be approved, and the category you file them under (marketing, utility, authentication) decides the price.
• RCS carries images, buttons and suggested replies, which changes what a "message" can be.
• Email allows anything, which is why its filters are so strict.

On SMS, a message that was delivered and ignored looks exactly like one that was delivered and read. WhatsApp and RCS give read receipts where the user allows them. Email open rates have been unreliable since privacy features such as Apple's Mail Privacy Protection (2021) began loading images automatically.

So any journey that waits for a reply must say what to do when nothing comes back — and the right wait differs by channel.

The penalty for getting it wrong differs too, and varies by country:

See sender registration around the world for the wider picture.

• SMS: allowed if you hold consent and your sender is registered in that market. Costs more in non-Latin scripts. Arrives in seconds. You will not know whether it was read.
• WhatsApp: needs a utility template unless the customer wrote to you in the last 24 hours. You get delivery and read receipts, and the customer can reply — often more valuable than the notification itself.
• Email: cheap, but competing with everything else in the inbox, and the open rate does not measure what you think.
• RCS: the same notice with a "track parcel" button where the handset and operator support it — and an SMS version for everyone else.

Four channels, one event, four designs.

The shortcut is still worth having. Just make the rules unavoidable:

• Check permission inside the send function. It should take an intent ("shipping update"), a contact and a consent basis, and refuse with a reason when the combination is not allowed.
• Look up capability at send time. Whether a number can receive RCS, or whether a WhatsApp window is open, changes from day to day. Decide when sending, cache briefly, and fall back rather than fail.
• Test it by calling it wrongly. If the answer is "it sends anyway and we hear about it from a complaint", the rules are decoration.

• Choosing a channel per message — the next step in this track.
• The 24-hour window — WhatsApp's key rule, explained.
• What RCS changed — the richer successor to SMS.
• Which channel for which need — every channel compared, with a primer for each, from SMS 101 and WhatsApp 101 to Email 101.