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From the call log you can atleast get the idea of delivery time and time taken to deliver it completely.

Try fetching it in a detailed manner

https://platform.devtest.ringcentral.com/restapi/v1.0/account/~/extension/~/call-log?view=Detailed and check the fax type there

Check for more details on fax log https://forums.developers.ringcentral.com/questions/144/fax-faq.html

The message-store entry has a "lastModifiedTime" which will be adjusted when the sending status changes, e.g. "Queued" to "Sent". However, this will be updated when the read status changes as well.

The best way to find fax delivery time is to query the "call-log" endpoint for "type" = "Fax". More information is available on this endpoint here:

https://developer.ringcentral.com/api-docs/latest/index.html#!#RefGetUserCallLog