How to stop random calls on my iPhone?

Without changing my number, How can I stop all of these random calls on my phone?

So one day I've bee expecting a call from someone, they were out of my area code, so the number area code was different from mine. I answered it just that one time! And then later on day I've gotten a couple of more calls that were random and out of my area code.

Ever since, I've been getting the most random numbers calling MY iPhone that it gotten to the point - two random numbers in a row would call me in the middle of class and leave me voicemails that last for 1-2 seconds.

I DON't LIKE THIS! Its getting annoying and annoying everyday, this never happened too me since that day a number called me out of my area code. Without me having to change me number how can I make it stop?

IPhone 6 suffered from water damage?

I got an iPhone 6 from a friend and she told me it suffered from water damage. She told me about how
- home button doesn't work (you could use assistive touch)
- There are these little lines on the screen
- sometimes phone turns blue and you have to wait 10 seconds before it restarts. I didn't mind because it wasn't that bad when I used it for a few days but she also told me when you lock it it won't turn back on until it completely dies. This morning, I accidentally locked it and it completely shut down and was not turning on. I asked my friend how I'll know if it died. She said you won't, but keep trying to hold the power button down once in a while to see if anything comes up.so, when the red lightning alert comes up to charge it, then I can charge it and it'll turn back on. But if nothing comes up and the screen is black, it didn't die yet. The phone was fully charged and she said it'll take a while to turn back on. Does anyone know how long it will take for the battery to die when the phone is off?

Added (1). it's been 13 hours. I've been counting LOL