I just bought an BlackBerry 7100v

& I realised a problem: when somebody tries 2 call me, it rings 1 time and then he get a bussy tone. My phone doesn't ring, but it displays that I have 1 miss call. I don't figure it out what is happening & I'm embarased

2 go 2 a service & 2 see that is from settings...

So, pleezzz, can somebody help me??? 10nx!