The short version
In India, a transactional SMS cannot simply be sent. The sender identity has to be registered, and so does the text of every message template, under the TRAI Distributed Ledger Technology framework. Until both exist, the message does not leave.
This surprises people because messaging looks like the easiest feature on a roadmap. It is not the code that is hard.
What is actually involved
An entity registration, tied to your legal identity. A sender header, the six characters a patient sees the message from, which is reviewed and can be rejected. Then a template for every distinct message: a one-time code is one template, an appointment reminder is another, a report-ready notification is a third.
Templates are approved individually and the sent text must match the registered text, with only the declared variables changing. A message you have not registered is a message you cannot send, however well the software works.
What it means for you as a buyer
When a vendor says SMS reminders are included, the useful question is whose DLT registration they run under and which templates are approved. If the answer involves you registering templates, that is calendar time on your implementation, not theirs.
The same shape of dependency governs WhatsApp: a Business Solution Provider, a verified business profile, and per-template approval from Meta. It is one reason we started that process before building the feature rather than during it.
Where we are
Our sender header is registered and active. The one-time-code template is approved. The remaining work is provider-side wiring and funding, after which one-time codes send for real, and every other message text needs its own registered template before it can go out.
So the honest statement today is: email sends, SMS does not send in production, WhatsApp does not exist, and there are no automated appointment reminders. We would rather write that sentence than let a buyer discover it in week two.