The Inbound tab of Settings holds two URLs where Heullo will call you, rather
than you calling Heullo.
The two URLs
The inbox receipt callback receives the sender, the receive date and time, and the
message text for each inbound message.
What your endpoint must do
Respond to a GET with OK. That is the whole contract.
Whitelisting
Receipts are sent from a fixed source address, which the tab displays so you can
allow it through a firewall. Take the value from the screen rather than from here,
so that it stays correct if it changes.
Each callback is attempted once. If your endpoint is unavailable at that
moment, the callback is not retried. Design your endpoint to accept requests
reliably, and treat the inbox in Heullo as the authoritative record of messages
received.
Configurable webhooks
For per-webhook configuration, request signing and a test facility, use the webhook
operations in the API. See the Inbound SMS section of the
API reference.
Verifying your endpoint
Send yourself a message and confirm your endpoint receives the callback. 
