Panasonic KX-FT21RS - Manual (Page 39)

6. The unit can copy, but the transmission and reception image is incorrect
(Long distance or international communication operation)
This depends widely on the transmission and reception capability of the other FAX unit and the line conditions.
The countermeasures for this unit are shown below.
Transmission Operation:
Set the transmitting speed to 4800BPS (service mode: code No. 717) or select the overseas mode.
Reception Operation:
If 80% or more of the reception is incorrect, set the receiving speed to 4800BPS. (Service mode: code No. 718)
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KX-FT21RS