You have just signed and accepted a new employment offer and now are ready to turn in your resignation and commence serving your notice period. However your manager is on leave or works remotely, then what do you do?
It is always best to give notice in person. If your manager will back in a day or two and you can afford to wait, then that perfect.
But that is not always the case. If your manager is working remotely or on vacation but reachable, sending a quick email or message like “Something has come up that I need to talk to you about. Do you have some time for a quick call?” And if you're disturbing him/her on vacation, apologize for the interruption.
Once you are on the phone or on a video call be really descriptive about why you came to the decision, which can usually get lost in translation.
You might also want to follow up with your manager when he/she is back in the office to go over a transition plan. That way, even if the timing is not good, you are taking responsibility to make the process as easy as possible on them.
If your boss is away and truly unavailable—for example, “On a holiday with little or no mobile coverage or email access”—the best would be to still call from your mobile and send a "Need to Speak Urgently" email or even approach their manager with whom “you can start to lay the groundwork.” And although this situation will not be liked by your manager (when access is gained), you can make the best of it by putting together a suitable transition plan.