[torbjorn]

In which country are you trying to use the fax machine, and which country is it initially sold?

Usually, adjustment of the transmit level is only possible on telecom equipment intended for use in the U.S. In other countries, the transmit level has to be fixed and it is not allowed to have any adjustment possibility for it.
I have encountered problems with faxes and modems connected via badly managed PABXs which often give a lot of gain or loss between the subscriber and the outside line.

For a fax machine, a lot of loss is usually better than too much gain, which might cause saturation, distortion and so on. It could be a good idea to try connecting an attenuator pad of perhaps 5 or 10 dB between the fax and the line, but then make sure that it does not create a DC path across the line or load down the ringing signal too much.

Another cause for problems might be that the fax machine is manufactured for another line impedance than is standardized for your country.
Traditionally, 600 ohms resistive impedance was used in a lot of countries. But today, various complex impedances are more common. For example, most european countries use 270 ohms in series with a RC network of 750 ohms parallel to 0,15 uF.
If the fax has a impedance that is not matched to the telephone network, the transmission will suffer from too much sidetone, i.e. "line echo".

[Justmanuals]

You'll need the service manual to make this adjustment.

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[zillah]

I am trying to use the fax machine that I bought from overseas (Panasonic model KX-FC379CX), when I tried to use it, it tells me that the transmission failed, the error report part of the error showed below :