So many people are not clear that they want a call-back. Often a hobbyist has privacy concerns and would not want us calling at the wrong time.
Unreturned phone calls do you make before you take the hint and give up?
Maybe two, if you misdialed? Lots of hot babes out there, don't waste your time! 
I agree. I wouldn't want to seem stalkerish anyway. But that's just me. She either got the message or she is not responsible enough for me to trust.
I wouldn't see some of my regulars if I only gave them one call and gave up..
So many people are not clear that they want a call-back. Often a hobbyist has privacy concerns and would not want us calling at the wrong time.
I thought that the OP was referring to making contact and not getting something back. These days - with cell phones - you know when someone has tried to contact you. Any reasonable person will specify what type of contact he likes if he has an issue.
It's not like we he's trying to court you. Persistence is a virtue in real life/dating (assuming it doesn't get stalkery...). In P4P, not so much.
One - and done. Reply or don't reply. No point in calling/texting/emailing more than once. Me - I'll give it a weekend or possibly day. After that - on to the next.
I agree. I am very specific about calling back before 5:30 pm. Sometimes this gives them an hour, sometimes four or more hours.
I have seen other posts where people say to keep trying. I just don't want to look like a stalke