Call forwarding from a toll free number to another toll free number often does not work due to the cost structure of toll free numbering system. When a call comes into a toll free number, the owner of the toll free number is responsible for paying the cost of the call. In a regular calling scenario, when a call is coming in from a non-toll free number to a toll free number, the billing is very easy to determine. However, when there are multiple toll free numbers in the call chain, each party tries to pass off the cost to the other party, and often toll free providers avoid this hassle by rejecting calls from other toll free numbers.
This behavior is inherent to the toll free phone number system and is not a specific to calls made with Twilio toll free numbers. Regretfully, Twilio is not able to guarantee all toll-free to toll-free calls will complete successfully. We recommend having Twilio toll free numbers forward to regular long code numbers in order to receive all incoming calls.