I'm not sure what the direct correlation is between the technique used to research a location and the technique used to gain permission to hunt the location. To me, they are two distinctly different processes.
My personal technique for finding potential hunt sites is to use a combination of historic maps, historic aerials, local history books, ancient newspapers, knowledge from old timers, Google Earth, etc to discover and flesh out potential detecting locations. Typically, I might have a list of a handful or even a couple dozen potential properties before I even start to consider finding out who owns them.
To echo what others have said, cold-calling the owner of a potential permission site on the telephone has extremely low odds of success. There's a very high probability that you will be treated like a telemarketer -
and in today's world that means you won't even get the opportunity to talk to them. When I get a phone call from a number I don't recognize or isn't in my "contacts" list, I simply don't answer and I let it go to voice mail - I'm pretty sure that's getting to be more and more of a common practice to avoid annoying telemarketing calls. So the property owner lets your call go to voice mail...now what? Do you leave a message? If so, now you're just talking at a machine. The "sales pitch" better be good, because chances are the person won't listen to the whole message before hitting "delete". And what if they don't call back? Do you call again, becoming a nuisance or a stalker? If you don't leave a message, how many times do you try again? Again, you risk becoming a pest or never breaking out of the "send unknown callers to voice mail" scenario - for me, if the unknown caller doesn't leave a message, it's a sure sign that it definitely wasn't important or worth my time!
But honestly, if all that wasn't enough to discourage phone calls for permission, isn't it much more difficult to try and secure a phone number than the address anyway?? When I'm ready to seek out a property owner for permission, I use a couple of different sources to get the owner name and address very quickly. First stop, especially if it's local, is the local county auditor's website. Many counties now have online mapping tools that allow you to access all of the information on a parcel that's considered public information, which includes a contact name and address - and you'll be amazed at how much information is considered public. A second tool I often use is a phone app called onX Hunt, which is fantastic when I'm on the road and I see a property that looks interesting -
see my post here for more detail about onX Hunt.
Anyway, once I have the name and address, it's time to talk directly to the owner in person about gaining permission. Folks have varying techniques and spiels that they use when "door knocking", but the main point is, a face-to-face meeting allows you to avoid the problems outlined above when trying to use the phone. You'll get a definite "yes" or "no" - they can't shuttle you to voice mail! And just as importantly, folks are less likely to say "no" if they meet you, see your passion for the hobby, and you can get them excited about it by showing them some examples of the types of things you find.