Tomb of Levistus is an eldritch invocation for warlocks from XGtE, p. 57 (emphasis mine):
As a reaction when you take damage, you can entomb yourself in ice, which melts away at the end of your next turn. [...] Immediately after you take the damage, [...], your speed is reduced to 0, and you are incapacitated. These effects, including any remaining temporary hit points, all end when the ice melts. [...]
I believe the intention is obviously that you are, well, "entombed in ice" and hence cannot move.
However, the spell Freedom of Movement states
You touch a willing creature. For the duration, the target’s movement is unaffected by difficult terrain, and spells and other magical effects can neither reduce the target’s speed nor cause the target to be paralyzed or restrained.
Clearly, Tomb of Levistus is a magical effect, so RAW I'd say you can still move. However, in my opinion, it makes no sense at all that you would be able to move and still gain all the other benefits from Tomb of Levistus, just because of one mid-level spell.
Are there any rulings/guidelines on similar cases / etc. that provide an indication of how to handle this situation? Or is it completely up to the DM how to handle the case?