ECM stands for Error Correction Mode. When ECM is active, fax pages are sent in blocks. If a block of fax pages fails the error check, the fax receiver will request that the sender re-transmit the block of pages until all blocks have been received successfully.
ECM automatically detects and corrects errors in the fax transmission process that are sometimes caused by telephone line noise. If ECM is disabled or unavailable, there is no way for the fax receiver to request that the sender re-transmit the part of the page (the page block) that has not been received correctly. Turning off ECM might improve the ability to send a fax successfully when the phone line quality is poor, but the image quality of the received fax might also be poor and/or missing parts of pages.
With handSIP, ECM is always enabled, providing the most reliable fax over IP experience available.