By default, TollFree numbers are not whitelisted for your account. You need to connect with Plivo to get your account & use case whitelisted. Post this verification process, you will be able to send SMS/MMS through TollFree numbers. Listed below is the process to enable SMS/MMS in your TollFree numbers.
To enable SMS/MMS in your Tollfree number:
Fill the required details in this form: https://docs.google.com/forms/u/1/d/e/1FAIpQLSfVR8ajPSsZpcfADMH0yHXDbmPO0s8opx8_kE0DLNYU6Yi0kA/viewform
Raise a ticket to Plivo-Support referring to the email id used to fill the above form.