You can leave a voice mail. I don’t answer to unknown number anymore due to heavy phone spam. My thought is "if it’s really important they will leave a message".
Thats interesting - I have two phones (with different numbers of course) and I almost get no spam calls (maybe a bit on the company phone but its still very low number). I must be doing something right here :)
Generally that seems like a good idea. In case of father from my story somehow this was not possible - I do not remember why - either he had voice mail turned off (it could have been paid service at that time) or he did not know how to use voice mail.