In this paper we study free boundary regularity in a parabolic two-phase problem below the continuous threshold. We consider unbounded domains ⊂ R n+1 assuming that ∂ separates R n+1 into two connected components 1 = and 2 = R n+1 \ . We furthermore assume that both 1 and 2 are parabolic NTA-domains, that ∂ is Ahlfors regular and for i ∈ {1, 2} we define ω i (X i ,t i , ·) to be the caloric measure at (X i ,t i ) ∈ i defined with respect to i . In the paper we make the additional assumption that ω i (X i ,t i , ·), for i ∈ {1, 2}, is absolutely continuous with respect to an appropriate surface measure σ on ∂ and that the Poisson kernels k i (X i
,t i , ·) = dω i (X i ,t i , ·)/dσ are such that log k i (X i ,t i , ·) ∈ VMO(dσ ).Our main result (Theorem 1) states that, under these assumptions, C r (X, t) ∩ ∂ is Reifenberg flat with vanishing constant whenever (X, t) ∈ ∂ and min{t 1 ,t 2 } > t + 4r 2 . This result has an important consequence (Theorem 3) stating that if the two-phase condition on the Poisson kernels is fulfilled, 1 and 2 are parabolic NTA-domains and ∂ is Ahlfors regular then if is close to being a chord arc domain with vanishing constant we can in fact conclude that is a chord arc domain with vanishing constant.