SUB-522 Error with Webhook Subscription

We always encounter this error when trying to make a webhook subscription with telephony sessions:

{"errorCode":"SUB-522","message":"WebHook responds with incorrect HTTP status. HTTP status is 400","errors":[{"errorCode":"SUB-522","message":"WebHook responds with incorrect HTTP status. HTTP status is 400","status":"400"}],"status":"400"}

with request:

{"eventFilters":["/restapi/v1.0/account/~/telephony/sessions"],"deliveryMode":{"transportType":"WebHook","address":".../chatapi/webhooks/ringcentralphonecall"}}

The strangest part is that our webhook code doesn't even seem to trigger. We also return the validation token, and we verified the address works using cURL. Could this be an issue with our account?

Notes:

- We are in sandbox

- App permissions: Call Control, Read Accounts, Ring Out, SMS, Webhook Subscriptions