Hi Jacky,
if in new MGW R4 you have declared all HO with LACs from legacy R99 MSC, then probably it is software issue in MGW. Declaring LACs and installing patch, Inter_MSC_HO_Incoming_Failure is solved in our case we had. I don’t know nothing about NSS, so I have to ask NSS guys and will give you more info what they did exactly. We (radio part) knew that is not radio pb and opened “trouble ticket”, after that they found software pb and the pb solved. During drive test measurement we saw that sometimes no handover were made between cells from legacy and cells from ngn even if HO were declared and near the ngn cell. From ngn cells to legacy cells it was OK. That’s all I know for the moment.