oeeehm.... correct me please if I'm wrong- just trying to give a small overview:
I think using asterisk with openwrt is still restricted to SIP connections only?
(other protocols that use network (tcp,udp,...) for transfering may also work - I'm no expert in VoIP).
If you want to use a traditional phone-line or ISDN for example make sure you keep the following in mind:
1. The kernel needs drivers for your hardware
2. Asterisk needs to be able to talk to the hardware (CAPI...)
So, most 2.6 kernels shipped with full distributions like debian, ubuntu, suse for example contain a lot of ISDN card drivers (FritzCard for example),
you can then install a CAPI and use asterisk with chan_capi for example (I'm currently running asterisk 1.4 on debian etch with a passive old PCI fritzcard (using avm capi) and it works fine..)
The Asus WL-500gP only has USB-ports, so you first have to make sure there are linux drivers for that USB-ISDN hardware, then you probably have to build your own OpenWRT kernel/image after you compiled that drivers for openwrt or create a "kmod..." package for them that others can also use... last but not least you may need a CAPI (I'm using the AVM capi on my debian), which you also have to build for OpenWRT and the asterisk chan_capi support... you may run in a lot of trouble when compiling all this for openwrt, because openwrt uses a different C "base-library" (uclibc) instead of libc that is used on full-distributions...
as you can see, there is much work to be done and I'm not sure wether it is possible or not at all... may be there are already some USB-ISDN drivers available as packages - anyway, that was the point when I decided to use a debian-etch system instead of openwrt for that....
oh.. by the way, another solution would be to use PCI-express ISDN cards, which are also available ( expansive ).. but the thing would be quite equal...