Sonic.net

I asked someone to send me a fax to my new FaxLine number, and the FaxLine History shows that the fax was received, but the fax was not sent to the recipient email address that I specified for my FaxLine number.

Is the fax stored somewhere, such that I can retrieve it or have it sent to another email address?

It does not seem the faxes are stored for later retrieval.
What probably happened was the email you provided was invalid or delivery failed or it went to a spam folder or something.
You can put multiple recipients. maybe use your sonic issued email address and try to send yourself a fax (not sure it's supported) or send a fax to that number from another number to confirm it works. Then ask for the fax to be sent again.

Not it's a labs feature so it's not officially fully "supported"

It works for me to send and receive faxes and i find it super convenient.
Thank Sonic.