I am a resident in Tokyo since October 2015 (until September 2017) and I use a pocket wifi. I don't have a Japanese phone number, so I use internet for all communications (calling and messaging with Line, Whatsapp, Messenger). This has never caused any trouble. People at work know they can contact me on Line,... Everytime I have to give up a number somewhere, I give the general phone number of the building where I live. For me it is convenient and I don't have to buy an expensive combined smartphone - phone contract (when I informed for this in Yodobashi Kamera, I was surprised by the high prices).
I bought the phone I wanted (LG G3) on Amazon.co.jp. I don't use a SIM card and carry my pocket wifi router all the time with me.
The pocket wifi I use is from WiMAX.
You have to sign a contract, the one I signed was for 2 years.
I pay around 4700 yen monthly (the first 3 months were at 3500 yen or so).
Advantages about this pocket wifi:
* A foreign credit card is acceptable.
* The connection is fast and most of the time I can stream without buffering.
* I don't have and don't need a phone card (although I understand that certain people probably need it for professional reasons).
* I use it for both my smartphone and laptop, so I don't need an extra internet connection for my computer.
A few drawbacks however:
* I have the "unlimited contract" of 2 years, which is actually not unlimited because you can use up to 3 gigabytes in 3 days. When you use more than 3 gb, it goes very slow for a day or so.
* If you stop the contract before the ending periode (2 years in my situation), you have to pay a fine (around 10000 yen if I remember well).
* The battery dies after 6-7 hours (it decharges more slowly when you use it less I have the impression) but I guess that is the same for all pocket routers, so on a day out you have to switch it off to save battery or take your charger with you. After 1 year of usage, my battery works still like one year ago (and I charge it every day).
* Outside of cities the reception is less reliable.
* In the subway tunnels I have no reception.
Buying a pocket wifi from someone is a little bit strange. He probably has some kind of contract, no? Be sure you don't have to pay some kind of fine.