Lumia 950 DS WiFi calling

[SirGader]

Hi! I am user of Lumia 950 Dual SIM RM-1118_13880. As per official specs this variant shall support both WiFi calling and VoLTE. Sadly, despite both being active on my account I cannot use either. However, when I pop the SIM to a single SIM variant RM-1104_15172 (which isn't operator-branded version either) WiFi calling works flawlessly. I am currently in an ongoing discussion with Microsoft - their operator claims that handset need operator-specific FW and he never saw Dual SIM with operator-specific FW. In addition, country variant of single SIM without operator-specific settings supported WiFi calling on my SIM.

Any idea why is that? It just doesn't make sense altogether.