After some testing I have found another SIP client, which shoud be used for SIP over IPv6 testing. The first one is Linhpone and the second one is IMS Boghe client. Right, Boghe is mainly IMS client, but may be used as an IETF SIP client, which allows to register and use an IETF SIP based network.
To make it working using IPv6 you have to change the following. Start the client and from the main menu choose Tools and Options.
In present days (start of the 2011) is not so easy to find SIP client which correctly support the using of IPv6. Up to now, with thanks of kamailio usr list, I found only one softphone and it is Linphone. It is quite simple softphone, but for IPv6 testing it is still enough.
To configure Linphone using IPv6 we have to change following: