"Thank you for calling."
Translation:Gracias por llamar.
Because "llamando" is the gerund, and llamar is the "infinitive".
This is a common source of confusion for English speakers, because we use "-ing" forms for both. I found a good article explaining it here... http://spanish.about.com/od/verbs/a/english_ing.htm
(Edited, with thanks to Fluent2B, who correctly pointed out my mistaking the gerund for the particple)
mind warp: "Here is the present-tense indicative conjugation of estar combined with a sample present participle to form the present progressive tense:
yo — Estoy escribiendo. — I am writing. tú — Estás escribiendo. — You are writing."
your article is a very good source for understanding ing in spanish.
Joe, I've noticed the same thing. The point here is that you can't trust them. They are not very contextually aware and thus often translate each word literally, one by one. That's usually good enough to get the overall gist, but not a precise or natural translation of the phrase or sentence.
I often conjecture to myself that the pull-downs in duo work like Google Translate, offering the possibilities or, perhaps, the best translations so far. I've also wondered if the pull-downs are generated by algorithms to include wrong answers that people have put down so that those wrong answers can be definitively weeded out.