LS400 Cellular Mobile Telephone

Hello Lexus enthousiasts,

I have a question regarding to my Lexus Cellular Mobile Telephone (1998).

Everything works perfect except i can't receive any incoming calls. When
somebody places a telephone call, i hear a click and that person then
receives the busy tone. So the Lexus phone seems to be engaged by another
call.
Within this telephone system there is a possibility to blockade incoming
calls, but this is set to "no-blockades".
I can't find any solution in the owner's manual.

This Cellular Phone was developed for Paneuropean GSM-systems, but perhaps
there are no basic differences with other country specifications.

Do you have any suggestions to solve this problem ?

Thanks in Advance.

Pos S.