I used to carry both a smartphone provided by the company I worked for plus a cellphone for private use.
At the time that I stopped working, there was no cheap SIM card supporting both voice and data so I bought a SIM free (unlocked) iPhone4s in Hong Kong and inserted a data SIM for internet use. I still had to also carry my cellphone. As soon as a cheap voice-plus-data SIM came on the market I got one and was happy to carry just one device.
Now tablets can use a data SIM and there's no need to carry a Mobile WiFi or use tethering, so it became more convenient. At the moment I use an iPhone, a tablet with a cellular SIM, and in case I need to use my PC outside the house, I have a Mobile WiFi.
Our communication charge is about 8000 yen a month, including home internet and telephone. I don't make a lot of phone calls because email and LINE are enough most of the time.
I still have some pay-phone cards so I sometimes look for a telephone booth to make a call, but today it is rather difficult to find one.