Yes the phone is set to prefer LTE but negotiates to HSPA if required due to the non-availability of an LTE connection. Making an outgoing call for example causes the phone to downgrade the connection to HSPA.
It appears that Virgin Mobile requires voice calls to downgrade to HSPA but something on the network is preventing this from happening for incoming calls.
Ok I've decided I have no choice but to cancel my contract because there doesn't seem to be a solution. There are people who have commented that they've been waiting months for a fix and I'm not willing to take that risk.