How do you write an SMS to a tag?

Writing the record

The record holds a destination number and a message. As with calls and mail, nothing is sent: the messaging application opens with the fields prepared and the user presses send.

Write the number in full international format for the same reason as a phone number tag. A locally formatted number fails for visitors, and visitors are usually who these tags are for.

Where support gets uneven

The destination number is handled reliably. The prefilled message body is the part with a history of inconsistency, between platforms, between messaging applications, and between versions.

The failure is partial rather than total: the messaging application opens with the right number and an empty message. Which is fine if the user knows what to write and useless if the whole design depended on a specific code being sent.

Test on the actual devices before building anything on the message body.

What it is good for

Short code interactions, where a number and a keyword together trigger something: a service that expects a message, a check in, a vote.

Situations with no data coverage but working cellular service, which still occur in rural areas and in buildings. A message goes out where a web page will not load.

And accessibility, for people who prefer messaging to calling and would rather not type a number.

When something else is better

If the goal is to collect structured information, a web form does it better and works on every phone. Messages arrive as free text and someone has to read them.

If the message body is essential to the interaction and cannot be typed by the user, the inconsistent support makes this a fragile choice. Design around a URL instead.

And if the recipient is a person rather than a service, consider that a tag sending them messages from strangers is a decision they should be part of.