What you've got is *normal* operation of the phone. Any cell phone does much similar things, on a similar schedule. The phone transmits "every once in a while", a "Here I am" notification to the network, letting it know that the subscriber is on-line/available, as well as a double-check on the transmit-power-level. needed to reach the cell tower, and a bunch of other 'housekeeping' details. In addition, before an incoming call 'rings', the cell system sends an "inquiry" to find out where you are, i.e. are you still in the 'last reported location', or somewhere else nearby, or 'not online at all'? Once you're "location verified", then it'll attempt the ring.
Now, _why_ this stuff is being picked up on the PC speakers -- that's a whole nuther story. Could be that you're on a different frequency band than prior phones. Could be a somewhat 'dirty' signal (relatively unlikely) from the phone, Could be any of a bunch of other things.