TWENTY pages?
Successfully faxing 5 pages is a challenge when using VoIP. The fax protocol is very unforgiving of connection quality. When fax error correction is ON, and an error is detected, it isn't sophisticated enough to correct just the error; it has to re-transmit the entire page, over and over, until it's successful. With EC OFF, it just gives up.
If you routinely have to send huge faxes, I'd recommend using a fax server service, whereby you scan and upload the document, and the fax service sends it for you, using high-quality connections.