LUIS gives 403 response with "Out of call volume quota." message even if I haven't reached 10,000 transactions free per month limit. My current usage is 1431 hits. I would also like to clarify that we are not hitting more than 5 calls a second. It's hardly one call a second.
Are you entirely certain that you are using the correct key, not the starter one? The starter one has a monthly limit of 1000 queries per endpoint per month, which possibly now would be exceeded with 1400+ queries.
See https://learn.microsoft.com/en-us/azure/cognitive-services/LUIS/luis-boundaries#key-limits
Authoring/Starter | 1 million/month | 5/second | 1 thousand/month | 5/second | Authoring your LUIS app
Versus the probably intended
Subscription | F0 | Free tier invalid | 10 thousand/month| 5/second | Querying your LUIS endpoint