Twilio imposes some rate restrictions for Programmable Fax. Read on for full details.
Twilio places no limitation on the rate at which projects can receive incoming faxes. Twilio will make an HTTP request to the url or service configured for the phone number each time a fax is received. Therefore, if you are expecting a large amount of concurrent inbound traffic, please make sure your server is capable of handling the load.
For help readying your Twilio number for incoming faxes, please see our article Configuring Phone Numbers to Receive Faxes.
Each Twilio number can request 1 originating outgoing fax per-second. Each page faxed will take 30-60 seconds to process on average. Sending a second fax to the same number, while your original fax's pages are still in transmission, will lead to a failed fax with the status "busy".