1. Use the standup
Given that you indicate you already have a standup, I would expect this person to be indicating there if they expect to need help to progress their tasks during the day. If they are doing this, you have an opportunity to say something along the lines of "take a read of my comments on the ticket and if you have any questions I can have a short call at chosen_time - perhaps straight before or after your typical lunch slot when your flow is broken anyway.
If they are not stating their blockers at standup, then coach them to do so - when they contact you remind them that they could have raised their issue earlier (or waited until the next standup if it is not urgent) and make sure that you are also modelling this behaviour - it's a lot easier for a junior to say they will need help if they see the seniors also asking for it some times (and none of us know everything).
And if they state in standup that they will be working on a ticket where you've made detailed comments, you also have an opportunity to head off the Zoom behaviour by letting them know that there are comments, and indicating your availability & preferred method of contact if they do get stuck (e.g. "Let me know in slack if you have any questions - I'm quite busy but will get back to you when I've had time to think about them" but remember, too, that a video call & screen share can be a lot quicker than typing for all but the most trivial questions).
2. Manage the calls
When they call you out of the blue, you have the option of not picking up. Rejecting the call and responding with a quick IM back saying "busy at the moment - can it wait?" will coach them that you are not obliged to prioritise their issue over your own, while also giving them the chance to tell you when it's genuinely something both urgent and unexpected, or schedule a time convenient to you if it can wait. Each time you answer the call and read the ticket to them, you reinforce their expectation that you always will.
Once you get onto a call, if there is a ticket with detailed comments, open it, screen share and ask them to be specific which part they are not clear about. Model the process of analysing a change request/bug report/whatever. Be open to the idea you could have been clearer, even if your more established team members don't have a problem - they have experience in your domain that this guy doesn't, or may just be used to some idiosyncracy in your writing style you don't notice yourself. Once this person gets used to the idea they need to be specific about what is not clear, they will take the time up front to think it through, and may work it out in the process. And if they haven't read the ticket before calling, politely end the call and ask them to read it and let you know in tomorrow's standup if they are still blocked needing clarification (back to 1!).
TL;DR - the best way to coach someone that you are not instantly available is to not act as though you are. But also remember that helping your team is part of your job, and cut some slack to a new guy who may just need a little coaching.