I changed from ddrwt with it’s GUI configuration to openwrt. Now my voip calls work again.
Linksys WRT54GL v1.2 settings: mtd erase nvram nvram set boot_wait=on nvram set boot_time=10 nvram set log_ipaddr=10.15.11.1 nvram commit uci set network.wan.proto=static uci set network.wan.type=bridge uci set network.wan.proto=static uci set network.wan.ipaddr=10.15.11.2 uci set network.wan.dns=80.68.95.249 uci set network.lan.ipaddr=10.15.11.2 uci set network.lan.type=bridge uci set network.lan.proto=static uci set network.lan.ipaddr=10.15.11.2 uci set network.lan.dns=80.68.95.249 uci set wireless.wl0=wifi-device uci set wireless.wl0.type=broadcom uci set wireless.wl0.channel=13 uci set wireless.wl0.agmode=11b uci set wireless.wl0.disabled=0 uci set wireless.cfg2=wifi-iface uci set wireless.cfg2.device=wl0 uci set wireless.cfg2.network=lan uci set wireless.cfg2.mode=ap uci set wireless.cfg2.ssid=free.imaginator.com uci set wireless.cfg2.encryption=none uci set wireless.cfg2.isolate=1 uci commit ifup wan ifup lan ipkg update ipkg install ntpclient ipkg remove dnsmasq wifi