The spell as written doesn't specify the method of choosing.
That said, it's entirely possible for the DM to allow asking a specific individual. Likewise, it's entirely possible for the DM to pick based upon story, randomness, patronage, or even simply being a jerk...
It's important to note that the spell needs to be useful - otherwise, it's not going to get used. All spells in the game are intended to be useful. Therefore, if the DM is picking, and picks someone who wouldn't know the answers, the DM is then clearly not in line with the intended meaning of the spell.
So, that leaves us falling back to choices.
Since it only appears on Wizard and Warlock class lists, one can glean that that rules out the clerical patron (Cleric, Druid, Paladin). Well, not entirely - multiclassing is possible, and some campaigns allow characters to have a patron deity without being a clerical caster. If they have one, it is quite likely to invoke their patron or their patron's lieutenants.
The lack of a specified method is most likely to allow for tailoring to one's campaign world. My own home campaign, for example, lacks the usual panoply, being populated instead with humorous homebrew deities. If they'd specified, I'd have to rewrite it anyway.
In Adventurer's League play, I'm not going to specify whom, I'm just going to answer the questions in a nearly omniscient fashion.
If your player specifies who they ask, it's fine to get them that direct line. If not, pick by whatever means you choose. And realize that a failed save may be "dialed the wrong being" or "called at a bad time."