I had an issue similar to this with 4.70 where an incomming calls via SIP to OS7200s which was sent via SPNET to OS7030 had no RBT on the callers end.

This was a SW issue and i got a T4.70z which fixed it.

Slighty different problem to what you have.

What VM do you have (20e or 20i)?

Maybe do a wireshark trace and see what you get?
You won't be able to decode the SPNET signalling but you will see the RTP packets in the MGI stream. Might be best to set the codec to G.711 so you can decode the rtp stream with wireshark.

You'll want to do a trace from both ends (ideally at the same time, but i know this isn't always easy).

Tech support will likely get you to upgrade, the latest seems to be 4.98 so you're 2 versions behind now.