"A" is correct here, and "the" would be odd.
Normally, "the" is the article used with an ordinal because what is being described is a specific item in a specific ordering of a set. There is no vagueness about what is being referred to.
In this specific example, however, there is, as yet, no such item to be referenced specifically. The sentence is conditional.
If more than twelve guests arrive, you will need to set up a third table.
In the preceding example, we do not talk about "the" third table because what is primarily being contemplated is a set of two tables. I do not say that "the third table" is wrong, but it would, in American English, be atypical.
In your example, it is not expected that someone will call more than once during an interval of fifteen minutes. If there were such an expectation, it would be proper to say in American English
When someone calls the second time ...
but even then it might better be to say
If someone calls for a second time
if there is no certainty about a second call.