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.
On identity tab we have to set up SIP account correctly:
Then on network tab you may use auto discovery of your SIP registrar/proxy with using of DNS, but of course you need correctly configured your DNS server. Do not forget check in IPv6 radio button to use IPv6 instead of IPv4
or you may manually specify your registrar