Use of the Siemens CX65 for sending faxes under Debian

I played a bit with my phone. When I use efax to send a PS-file (or TIFF-file) to the destination via CX65, it starts dialing but then immediately stops with "busy" signal. The phone shows message "no connection", and the efax returns

efax: 06:14 Error: dial command failed
efax: 06:14 failed -> faxtest1.tiff
efax: 06:15 done, returning 2 (unrecoverable error)

I suspect, it has something to do with commands of the modem. But I need more time to investigate the thing. Any ideas?