failure during first LCP Packets

I am connecting to a Endres and Hauser Fieldgate modem using pppd 2.4.1 and capiplugin 1.31 or an analog modem from behind a telephone system. Sometimes it works flawlessly, but sometimes the remote modem seems not to hear the pppd's answer. It sends the CHAP or IPCP packet several times, and pppd answers it every time, but it does not lead to success.
It is the same behaviour with several remote modems of the same type and from different linux systems.
There are indices that this behaviour does not occur, when calling the remote modem from outside the company, but I didn't verify that so long. Does anybody know what may be going on here, and has a hint how to work around?

Thilo