interfaces in sipmsg.log

Hello,

does anybody know what [256:0] in SIP methods below mean? I captured a 'notice' level sipd log. I need to know what the ingress and egress interfaces are.

I expect a following scenario. INVITE hits M00:0 and it is sent to M10:0. The AS should route the call back to the SBC, but now the INVITE should hit M01:0. But instead of something like [0:1] there's [256:0].

Why? What does it mean?? Pls note the 10.5.21.70 is the public interface and the 10.5.14.101 is the private interface of the SBC. The 10.5.13.199 is the SBC's private interface for outgoing trunk and 10.5.21.71 is the public interface of the SIP trunk. Both public interfaces are configured on M00:0.