All you need is a phone that has texting capabilities as some women tend to use texts rather than voice.  Very easy to get one at Walmart. I have a $20.00 tracfone.

Now whether you want to spend extra money on a phone totally depends on you and your hobbying patterns, along with your risk tolerance. Keep in mind that the activity is illegal in US. If things go wrong with the provider, your phone number can find its way on Blacklist boards. Can you afford to have your phone number leak on the Internet?  Some high profile people can't.
Also, do you hobby with agencies? If agencies get busted, and they get busted a lot, LE will and always does confiscate PCs and cell phones.  If your phone number is in their contact list, you can land on the Internet. Just search Desert Divas in google.

Some people like to take the extra step and get a hobby phone. If I were to advice you, I would say get one. Some people don't give a shit either way.  

Which one are you, only you can decide.

Cheap disposable hobby phone that bought with cash, refilled with cash (when you're in the hobby spirit) and can be thrown away at the first time of trouble or lost without a second thought. Use this phone exclusively for voice or text contact with a provider.

Your regular phone with a data plan you can use for emailing if necessary up to the time when email is no longer viable and then switch to voice or text on your hobby phone.

When it's appointment time, you can leave everything but cash and your hobby phone in the car. If you get your ass beat by a pimp or stopped by a cop. You're lose is minimal. Oh don't forget to clear the phone's memory after each appt.

10 years. It costs $20 every 3 months to "top up". The phone and the top-up cards are bought with cash and registered online with a public computer (library, health club, etc.) in john doe's name.

Like others have said...  20 bucks, bought with cash.  20 bucks every 3 months for service, you can add service/minutes through the handset, you don't need an online account.  If you can afford to play, you can afford the phone.  If you re-up before the service expires, the minutes roll over (on TF).  Shut it off when not needed.  This is my second TF...  a couple of years old now...  holds a charge well when not in use, much better than the older one.  It's a flip phone like my regular phone, so I swap them in my belt carrier when on a 'date'.  
If you ever change your mind, the deniability & ability to walk away with no paper trail, is priceless.  
Some guys put fake male names to numbers in their phone book.  I prefer to clear the history while I'm with her.  But it means I need internet access to look up numbers.

I would not get a second phone.  I have no SO, and no one ever uses my phone but me, so I do not have one.  There are those out there who will tell you it's a necessity, I just don't agree under all circumstances.  If you are fairly young and might become involved with a sig other, then it might be worthwhile to have it.  For me, since the only ladies I have any contact with are providers, it's an expense I choose to forgo.  

Previous posters are right, you do not need a high end phone for a hobby phone.  The cheap, loadable ones are all you need.

Swim

If your main phone is a smartphone, you can get "google voice" on it. It will be a seperate phone number with texting and voice capabilities. It is free. You can make calls from that number (but on your regular phone) and the google number is the one that shows up on caller ID.

Other possible options.that I can think of would be wifi base, but what if I cannot get wifi?  So that's out.

I do not live alone.  If found out I have two phones, that would raise questions.    I currently have a dumbphone.

I do miss having a smartphone.  So I was originally thinking of upgrading to an imported dual sim smartphone, to bypass dataplan? (since I can do everything I want via wifi and not data).  And selecting witch sim card/phone number and do it that way.

Lets say I go to best buy and get a tracfone.  How do you refill anonymously?  I cannot just buy a simscard?  

The downside of Google voice is that Google knows who you are.  If the shit does hit the fan a la Desert Divas, LE may be able to get those records.  That's the beauty of the burners.  If you are careful, they really are anonymous.

As far as the worry about a discovery of two phones, it is a concern among those of us that are married.  My rule is that the phone is kept at my office.  If for some reason it does end up home with me, I keep the phone and the battery in separate locations.  If the phone is found then I just claim that I found it walking home and was going to see if I could get it running.  With no battery it cant even be turned on.  Previous advice about clearing contacts, messages, and logs immediately following appointment is so very important.

Bottom line: Never use your personal phone number.  Plenty of flaky shit goes down here, more than just LE.  Providers flake out, their SOs find out and start calling, jealous scary clients find your number in their ATF phone, etc.  You dont want that stuff coming in on your regular phone.