It is a conjunction. Two things happen in the example sentence: first arrive home, and then call, refered to in two clauses and the when connects them.
However, it is a subordinating conjunction (sometimes called a "trigger word") introducing an adverbial clause, and on the principle that words introducing new information take a strong pronunciation the vowel is not reduced to a schwa.
As, when and while are conjunctions. In some uses as, when and while can mean the same, but they can also have slightly different meanings. We use them to introduce subordinate clauses.(From Cambridge "English Grammar Today")
A conjunction (also called a connective) is a word such as and, because, but, for, if, or, and when. Conjunctions are used to connect phrases, clauses, and sentences. (From Oxford Dictionary)
In English, there are lots of subordinating conjunctions, but the most common ones, along with a few examples of how subordinating conjunctions are used, are as follows:
when - “When I see you smile, I can face the world”
(From Your Dictionary)
As well as the coordinating conjunctions noted and explained above, there is a whole raft of subordinating conjunctions. Some of them are:
(From Bristol University Improve Your Writing)
Subordinating conjunctions are used to join independent clauses to make complex sentences. The subordinating conjunctions are as follows: after, although, as, as if, because, before, even if, even though, if, if only, rather than, since, that, though, unless, until, when, where, whereas, wherever, whether, which, and while. (From Purdue OWL)