There are several reasons that I sometimes don't answer my phone including but not limited to:
1. It is dead.
2. I unintentionally left it in the car.
3. I intentionally left it in the car because I am in church or at a doctor's appointment or somewhere else I do not want it to ring.
4. I am out and I accidently left it at home.
5. It is in another room and I do not hear it ringing.
6. It is in another room and I do hear it ringing but do not get to it fast enough.
7. It is in another room and I hear it ringing quietly and cannot tell what room it is coming from. In these circumstances it often weirdly feels like the sound is coming from every room.
8. It is in another room and I hear it ringing but can not get to it because I am in the middle of something like feeding or rocking a baby.
9. I do not know where it is.
10. I have company or am out with someone and it would be rude to answer the phone.
11. I do not recognize the number on the caller ID.
12. I do not want to talk to the person calling. (This I assure you, never applies to you.)
13. I do want to talk to the person calling, but do not have time right now to devote to the call.
14. The person calling is awkward to talk to, so I would rather just let them leave a message.
15. I know what the person calling wants and I do not yet have an answer for them.
16. I suspect the person calling is looking for someone else who is not with me.
17. I am in the middle of a serious conversation.
18. I am watching 24.