Exactly. In this case, the rule is simple: "llamar" always needs the preposition "a" when referring to the receiver of the call.
There are another couple of frequent prepositions: "por" and "en", a couple of examples: - llamar por teléfono, por radio, por megafonía: call on the phone, by radio, by PA - llamar en vano: call in vain
@seamus64: 'they will call at the hospital' meaning "they will visit the hospital": visitarán el hospital. [Edited]
That's a figurative sense of 'call' (from when you'd show up at someone's house and call out to see whether anyone was home), that's specific to English, as far as I can tell. At any rate, I'm pretty sure 'llamar' can't be used in that sense, so you'd have to say something like "ellos van a pasar por el hospital". Could a native speaker confirm please?
Because in this case "a" works as a preposition and you need to used it in order to create a connection between the words, otherwise this would be understood as two different sentences with no practical sense as "Ellos van a llamar - El hospital". So you use "a" and using "a" preposition + "el" goes "al" as a contraction that only works with "a"+"el", like:
A + el + supermercado = al supermercado = to the supermarket
A + el + banco = al banco = to the bank
A + el + hotel = al hotel = to the hotel
A + la + casa = a la casa = to the house (note that it only works with a+el).
Hope i was helpful :)