[Android Central Question]

I want to avoid all incoming calls and at the same time I want to surf internet on my Android smartphone

[ManiacJoe]

You will probably need to put the smartphone into "do not disturb" mode and set the "allow phone calls from" to "no one".
Since the primary purpose of the smartphone is to be a phone, you will need to jump through some hoops to turn off the phone part.

If you have wifi available, you might also be able to turn off the mobile data to block the phone calls as long as "wifi calling" is not turned on.

[Dano1200r]

Unless you are using phone over Wi-Fi, like AT&T, simply turn off your data and leave your wifi on.

[Rukbat]

Or turn on airplane mode, then turn on wifi or mobile data. That would depend on the phone - some phones will do that, some will turn off airplane mode, and some won't allow you to turn wifi or mobile data on when you're in airplane mode. (Mobile data is used to access the internet, but on most phones it's not used to handle phone calls, even though some carriers, like AT&T, use 3G data for phone calls.)

But T-Mobile will handle calls through wifi, so DND is about your only hope.