When you are on a Priority Hour, you should answer the call from the student at least once.
If he/she is clearly not serious or is causing problems, you can very politely suggest they call you back when they are ready to learn English and say goodbye. If the student continues to call you, you can either give them another chance, or you can ignore the call. As long as you have answered the student's call once during the Priority Hour, avoiding their calls during the rest of the PH will not be counted as "missed calls" for you.
For example, if you chat with John once during your PH and then ignore him 2 more times after that, we will not count as you as having missed any of John's calls.