I just installed the asterisk13 packages onto a 17.01.4 system. I believe I have everything I need, except that I can't find a package containing chan_motif, which is needed for a Google Voice trunk. I do have res_xmpp, the other half of the equation, but no chan_motif.
I'm not sure exactly what path the asterisk13 stuff takes to get into the opkg repository. It looks like this github repository is involved: https://github.com/openwrt/telephony. This 2017 closed issue https://github.com/openwrt/telephony/issues/110 asks about missing chan_motif and res_xmpp. The repo owner, @jslachta, says "should be fixed by now".
Anyhow, I just wonder if I am looking in the wrong place or something. (Thanks to all involved in OpenWRT/LEDE and the bringing of asterisk13 to it.