I thought it would be "τον" for masculine. Is this one of those exceptions or do some nouns become regarded as neutral sometimes?
According to the grammar http://bit.ly/1tftFRK definite or indefinite masculine articles should keep the -ν ending, even if the next letter is not a vowel or one consonant from this list:
κ π τ μπ ντ γκ τσ τζ ξ ψ
I think that this was just a proposal from Babiniotis (a greek linguist) http://www.lexicon.gr/keimena/ni_tono.php, but not yet an official rule.
So, if the next word begins with a consonant like :
β γ δ ζ θ λ μ ν ρ σ φ χ
then the article drops the final -ν. So ye, the right answer is the proposed above "Αυτοί καλούν το γιατρό" (without the final -ν).