The short answer
Two tiers
Your company's standard API key sends texts inside an included allowance of 20 messages a minute and 200 a day. A send over either limit is answered 429 and not sent. Volume above that runs on a metered messaging key, switched on per company under Textdock, API Messaging, at two cents per outgoing message, SMS and MMS alike, with incoming messages free. Ordinary texting in Textdock, in the inbox and by email stays included; the metered paths are this one and text blasts.
How a message is counted
Plain English, numbers and ordinary punctuation fit 160 characters in one message and 153 per part once it splits. One character outside that alphabet, an emoji, a curly quote pasted from Word, or Hebrew or Yiddish text, switches the whole message to the wide alphabet: 70 characters in one message and 67 per part. An emoji costs twice over, because it makes the switch and then takes the room of two characters. Ten characters (^ { } \ [ ~ ] | € and form feed) cost two units each, so the braces on a merge field cost four before any text. A picture message counts as one message whatever its text length. Paste a real message into the sampler on the API Messaging page and it shows the count and the price; Test it does a dry run that validates and sends nothing.
What is billed
A send that never reached a carrier is recorded but not billed. A message a carrier takes and then rejects is billed.
The keys
One key per integrator, not one per company. Each carries its own mandatory IP allow-list, its own usage counter, its own log, and its own on-off switch, and regenerating one leaves the others working. Each also carries two limits set by us and shown read-only: segments per minute and messages per day, counted in different units on purpose because the carriers count them that way. Anything over the per-minute rate is accepted, queued and sent in order, so your system does not have to pace itself.
A metered key can send and read text messages, fetch its own attachments, and manage webhooks for the three message events. It cannot reach calls, contacts or faxes; those need the standard key. See REST API basics and webhooks.
When messages are late
Delivery is paced per phone carrier, and your ordinary texting shares that room. If one carrier's daily room fills up, messages to that carrier wait for the next day while everything else keeps going, so a full day's batch is not guaranteed to land the same day.
Rules
The API is not a marketing tool. Turning it on means agreeing to text only people who shared their number and agreed to be texted, and to run no purchased, rented or scraped lists. The number the messages come from has to be registered; see 10DLC registration. The API Messaging page is the one Textdock page a customer login can reach; it needs the Company Details permission.