We have two sites connected via MPLS for spnet. Problem that we are having is when customer tries to transfer a call from site A to site B the call drops off and MGI cards at each site get a network error alarm and seem to reboot.
Internal calls work fine it is only transferring an external call (they have a PRI service)
Both system are 7100 4.7 software, they are set to private with public with private addresses entered in 838.
When i set systems to do spnet via the internet problems goes away.

What firmware are the mgi cards running?

There is an issue with 1.28 firmware which causes the mig16/64 cards to reboot when ip traffic gets a icmp gateway redirect from the gateway on a private network (eg when you have voice traffic going to a different subnet to the one the mgi card is in). I've only seen it with ip/wip/sip handsets but it could affect spnet as well.

The traffic is sent differently when using public so the issue doesn't happen

Downgrade to 1.26 and the problem should go away.

One site is running an OAS card v2.03, and the other site is running Mgi v1.28, i have a spare OAS card, might just swap it over and give it a try.

I downgraded the card and still have issues, it seems its just the mgi part of things that isnt working between sites, i had a phone forwarded to voicemail to test with. Phone rings and gets answered by the voicemail but have no speech both ways.

Why are they private with public? Do you have sip trunks?

I'm going to sound like plant tech support but have you done a wireshark trace? Since you can replicate the issue you well see what is sent to/from the mgi card at the time of the reset

Yes Private w public is set so that the sip trunks will work, I will have to try and get on site to do some testing, it is quite hard to make a time as when the fault occurs it cuts of all their calls.

I've been told (and seen) sip trunks work in private only and with incorrect public ip.

Give that a shot