How do I reduce delay between trunk answering and extension ringing

I have elastix 1.6.7 running on a dedicated box (Dell XPS, Intel Celeron 2.6GHz, plenty of memory) and a TDM400P clone card with 2 FXO and 2 FXS. I also have a number of SIP phones. All this is in my Home office. The two trunks are connected to separate BT lines: one for home and one for the business.

All seems to be working OK. Inbound calls are routed straight to a call group that rings a subset of the internal phones: depending on which inbound line is ringing.

The problem is the delay between the call coming in and the extensions starting to ring. This is of the order of 2-3 rings (as seen from the outside and from phones that are connected directly to the outside trunks.

The flash PBX screen shows the appropriate trunk ringing straight away but it takes too long for Asterisk to process the call.

Is there any way to speed this up?