Legal notice from Suzuki Law office?

I was offered free internet by Softbank Executive for 6 months. But after 3 months only they discontinue the service. i returned the router that came with free internet connection. Now i have received a notice from Suzuki Law Office demanding to pay JPY 73520.

I don't understand why i am receiving this communication. I don't have any pending bills of SoftBank still i have received it.

Please guide what should i do.

Maybe you should go to softbank and ask why this bill is coming from.

You got an offer for 6 months free internet (free does not exist, there are always small letters) and it is discontinued than it may mean you stopped earlier. But you mention that they (softbank) discontinue.
I do not understand this part.

https://www.suzukiyasuyuki-lawoffice.com/index.html

1. when the letter says "suzuki law office", 鈴木法律事務所, it is fake.
when the letter says "suzuki yasuyuki law office", 鈴木康之法律事務所, it is genuine.

2. when the phone number is 050-5373-7838, it is fake.

3.when there is no building name in their address, it is fake.

really there are fake invoices..... that is scary than.