I don't normally answer my phone when it rings. I sometimes leave it off the charger until the battery runs down and remember to plug it in a couple days later. Even when it is charged, I don't carry it around with me, and I prefer to let it ring and look up the number to find out who it was if I don't have the number programmed into my phone already. If it's someone I want to talk to and I get a missed call, I'll call them back, but I don't always notice right away, so it could easily be several hours later.
So I wouldn't take it personally, unless they respond quickly to everyone else.
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"A dead thing can go with the stream, but only a living thing can go against it." --G. K. Chesterton