This means that VoLTE is not operational; otherwise it would remain on 4G/LTE during the call.
Do you have the BM818 Tool installed, with VoLTE enabled?
If you already have it enabled and calls are still dropping to 2G, then either your carrier isn’t implementing VoLTE on their network (or the foreign network isn’t doing so for the BT SIM*), or the carrier isn’t implementing VoLTE for the L5 (actually the BM818 modem, which is in common use in the automobile industry).
*Since you’ve used the SIM successfully in other phones, you can probably rule that out.