In order to get a recessive phenotype, both parents genotype must contain at least one recessive allele, i.e. parents must be either be both heterozygous, or one heterozygous and the other homozygous recessive, or both homozygous recessive. (I.e. Tt+Tt, Tt+tt, or tt+tt)
In this particular case, since both parents have dominant phenotype, so they must be both heterozygous for Tall, or both Tt.
