Panasonic KX-FT21RS - Manual (Page 41)

TX=TRANSMISSION RX=RECEPTION
Most fax communication problems can be resolved by the following steps.
a. Change the transmit level. (Service code: 596, refer to 2.4.4. Service Function Table.)
b. Change the TX speed/RX speed. (Service code: 717/718, refer to 2.4.4. Service Function Table.)
If the problem remains, see the next page.
41
KX-FT21RS