Iaxmodem configuration

I have running Asterisk with iaxmodems and Hylafax on an ISDN-line for outgoing faxes.

Because of a lot of undeliverable faxes our provider has checked his logfiles ans find out, that we use a disconnect time of only 30 seconds and ISDN-Service: “Speech”.

Our provider recommend to change this to 45 or 60 seconds and ISDN-Service to “3,1 kHz Audio”

But I do not find where I can change it. Please can anybody help me?

Christian