Thanks for all the help! The solution was:

turn off bsddeflate with the pppd switch "nodeflate" on one line in
/etc/ppp/options.

The modem is an isdn terminal adapter, i. e. has the speed of isdn but
acts like an external modem. I recalled that on another machine running
GNU/Linux, pppd was acting strangely, spouting error messages. Turning
off bsddeflate solved that problem. Hope this might help others with the
same problem.

Will post another config question on the text console.

Lucien
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