[Serusers] 183 session progress

I have a doubt with a scenario, where a call is done between two APX/MAX-TNT Lucent Gateways(GW):

Gw A SER GW B | INVITE-> | | PSTN A | <-100 | | PSTN B side side | | INVITE-> | | | <-183 w/SDP | | <-183 w/SDP | | | | | | | |

Then, the call downs by some reason as timeout. I just don't understand well the meaning of 183. AFAIK, when it happens, it should or could be an 'early' rtp flow from B that should be audible for A (for example, A could expect a play message from PSTN B side operator). Is that correct? Could someone clarify me this concept?

Thank you!

PS: does someone use ever APX/MAX-TNT Lucent gateways? I have some doubts about his configuration.

Jorge