[Serusers] IPv6 configuration problem

Hi, I am running ser-0.8.14 on a Linux box with the network card configured with both IPv4 (a.b.c.d) and IPv6 addresses (1:2:3:4:5:6:7:8). To make the SER listen to both IPv4 and IPv6 incoming calls, I have the following lines in ser.cfg: listen=a.b.c.d 1:2:3:4:5:6:7:8 port=5060 However, the SER is not responding to any incoming requests. For example, when my IPv4 clients (tried both Messenger and Xlite) attempt to Register, there is no response from SER server. If I remove the "listen" line from ser.cfg, the SER server works fine just as normal. Can anyone please point out what is happening? A working IPv6 setup example will be great if you can provide one (including ser.cfg, clients used, and relevant settings for the client). Thanks, Haipeng