Consider how this will impact the negotiation between clients and servers. If your B2BUA thinks calls are coming in from IPV4 (instead of IPV6), then it may not offer IPV6 in the invite. Similarly, if the client thinks it's connecting to your SIP infrastructure via V6, then it'll likely offer V6 in the invite which your B2BUA can't accept. ICE can probably help here, by offering both V4 and V6 candidates, but that's going to depend on your client/server setups. Unless you want Kamailio to proxy media s well, in which case you may be able to escape this, but by adding complexity you Kamailio.

If at all possible, I'd try to preserve the source protocol through to the end. If this is something you already thought of though, great! Would be good to hear what you come up with here.

Best,

Colin