Canon PIXMA MX310 All-In-One Printer "memory low fax machine mean" Questions

select [ON] and then press . To store received faxes in memory when ink is low or out, press the left or right arrow to select [OFF] and then press . Press the repeatedly to return to Fax standby mode

memory black ink low - recommened new c-crt. I cant recieve any faxes change the ink cartidge After faxing I get recom. new c-crt. What sure I do? cj My printer says recommend new c-crt. What is c-crt