OK, I do have a SO...  but even if I was not...  I would still suggest that precautions be followed including secure e-mail, hobby phone, etc.  Leaving tracks is just plain sloppy...  when I started this hobby I promised myself I'd not do anything stupid.  Well, I was lucky & luck is not to be relied on!

Your status may change...  employers & coworkers could find out...  tracks last a long time, if not forever.  If you can afford to hobby, you can afford a free e-mail, a hobby phone, and TER VIP.
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turn it off and not have to worry about the indiscreet call back.

Also, you may very well find yourself in a relationship in the near or not so distant future in which you wish you had a separate number.

Finally, if you set the activation process of the phone up correctly, the number may not be able to be traced back to you if LE or a jealous boyfriend finds the number.

Just some thoughts, but ultimately of course, it is up to you.

Being single, with no SO, my cell phone is my only cell phone.  Protection from discovery by wife or gf is probably the top reason for having a hobby phone, but I have no such need.  If you are in my situation, it's a decision only you can make.  Providers know my number and some even call me to just chat from time to time, which I enjoy.  I don't want to carry two phones and then explain why to a friend that is unaware of my hobby life.  

Providers run the same risks of discovery that we do if they are in a relationship, and I'd bet many of them have a personal phone and a 'business' phone.  At least one or two providers I know, if you call the number in their ad or on their website, you will get their assistant, not them.

Swim

In most cases, married or otherwise attached guys would be better off to use one, but my situation doesn't warrant it. I'm reasonably sure that my wife is aware of my extra-cirricular activities. It's a little different situation than most.

Re: what do u guys think about hobby phones?

You really don't need to purchase another phone, just purchase a SIM card from the same carrier and swap cards when necessary. You can even purchase a SIM card that is pay as you go. The SIM card is easy to hide in a wallet or other place.

I don't use one because my wife is not nosy, I have a good reason to have many ladies in my address book and phone log, due to my occupation, and the ladies I do have on there are very discrete, so if my wife did call, they'd know what to do and say. But, I could see where others, who have a SO, would want a hobby phone.

I think of it as an insurance policy.  Even if you don't have an SO or employer to worry about it can protect you from the rare event.  As someone mentioned you may have a provider with a jealous SO or maybe meet a provider who later has LE problems.  It is easy to just dump the phone in the trash and get another.  

On the LE front, you may recall that the DC Madam (RIP) released her phone records.  You could be embarrassed if a similar situation happened and your personal cell number was plastered on the Internet waiting for your friends or a background check to google up.

It's a cheap insurance policy.