Contacts are optional — you can send to raw numbers — but storing them lets you
send to a group instead of assembling a list on every request.
Endpoints
International format, e.g. 255766298542.
Groups
Group membership is managed separately:
You are responsible for having a lawful basis to message everyone on your lists, and
for honouring opt-out requests. Uploading a purchased or scraped list breaches the
acceptable use rules and can get your account
suspended.