I have an ISDN trunk connected as my main lines in place of the old PSTN.

The PSTN would allow the DTMF key tones to be heard when dialling, but with ISDN, although you hear dial tone when you select a line, as soon as you dial a digit, the dial tone vanishes, and key presses do not emit the DTMF tone in the ear piece.

The calls go though just fine, but some staff find it odd with no dial tone and DTMF and either end the call to start again, or mis-dial!

Is there an option to switch this?

It is almost like a least cost routing thing, where the DTMF doesn't go to the line immediately, although I can't see any settings set that would effect this.

Is this just the way the NEC Topaz/xn120 works?