SP1000SF Unable to Fax Out But Able to Receive

I'm currently having a problem with this model where the fax can't be sent out but able to receive. I have tried using the fax line to call out and there is no problem. Also, i tried the on-hook-dial, there is dialing sound. But once I tried to send a fax out, the fax will not go through. What seems to be the problem? I advised customer to check their fax line with the service provider and responded that the fax line is fine.

There is the envelope sign on in the screen. What does it mean? Is it the memory is full causing this to happen?