Dear, Mouf! As I know there is no signalling relationship between BSC and MGW. All signalling is t*rminated in MSC-server and handled by him. As I know the Ericsson MGW is coupled with Signalling Gateway (SG). SG function is transport conversion from SS7 (MTP) to Sigtran. But an Application Part signalling (BSSAP) is delivered without conversion from BSC to MSC-server. The effect of signalling connection with two MGW is the reserve way for signalling or signalling load sharing.
Pan is correct the UMG is just the bearer interface and the converter(SG).It converts the TDM TO IP AND IP TO TDM and send all the messages to the MSC server, It doesn’t matter whether the BSC traffic goes through a single or multiple MGW the destination is the MSC Server. MGW doesn’t process the signalling
I think what Mouf refers to is the feature called A-Flex. The purpose is to increase the availability for the BSS / RAN network, in case one BSC MGW fails to work. To avoid a complete LAC to be out, the A-flex offers network connectivity and voila: no BSS downtime in case of MGW outage.