Matching MO and MT-REPLY in struct Msg

Hi,

First question while working on CPA.

For this CPA, there is a different between a MT-Push and MT-Reply, in 
that if its a MT-Reply, a transaction_id need to be mapped back to the 
original MO sent from the SMSC.

So the scenario is,
1. One MO comes in from SMSC, it contains a transaction ID, I save all 
the sender, receiver, msg... stuffs into a struct Msg, and send it over 
the bb_smscconn_receive.

2. When I get back the MT-Reply from the callback send_msg, how can I 
retrieve back the transaction_id that I am supposed to reply to the SMSC?

I have checked thru the source codes, although that are a lot of 
parameters in struct Msg/Sms that I can use, but none of those seemed to 
be duplicated for the MT-Reply message from the MO Message. Only the 
sender, receiver, msgdata, and time are set for the MT-Reply Message.

Is there anyway that I can save something onto the struct Msg, and 
retrieve it back so that when my send_msg callback is called, I can know 
that if its a MT-Reply, I need to get the transaction_id from it.

I could have just hacked smsbox, but wondering if there is a _clean_ way 
to get around this? Shouldn't it be that when a reply_msg is 
constructed, the whole msg should be duplicated, and then just replacing 
the msgdata and also swapping the sender and receiver? That would make 
it easier for the smsc_xxx implementations to save whatever information 
that is required to do a MT-Reply mapping.

Cheers and Thanks in Advance,
Phuah