It's a very common sound but I've never thought to question why it happens.
I assume that you have a GSM phone.
When the GSM protocol was developed, they overlooked the fact that a "frame rate signal" in the protocol caused modulation of the RF signal at a consistent audio frequency rate with very clean consistent periods with and without RF. The GSM system was so important and so much had been spent on it that the allowed this defect to continue even though it would have been unacceptable in any equipment submitted for approval by the normal means.
That this was a complete mistake and completely unknown until late in the development is described here in Chapter 22 - Unintended Consequences of "Inside the Mobile Revolution" - a history of GSM. The writer is 'Stephen Temple CBE*', one of the few big-fish in the introduction of GSM**, arguably the UK's leading 'player' and, the text leads you to think, responsible for GSM being much of what it became.
He makes a very clear and open statement about what happened - and how it was unacceptably bad technically BUT unable to be stopped politically.
"INSIDE THE MOBILE REVOLUTION" is
From the foreword:
- The GSM 1800 MHz (PCN) element was added a few years later when the UK's DTI was at the leading edge of telecommunications liberalisation and I propelled it to be the first credible entity to set out the vision of the personal mobile phone as a consumer product (‘Phones on the Move’) and put forward the conditions to make it happen. My election as Chairman of the Technical Assembly of the European Telecommunications standards Institute in 1988 then provided an excellent vantage point as GSM went through its transition from a policy to a new born infant digital mobile industry. All this left me sitting on a unique (although in parts partial) account of how GSM came to be the mobile radio equivalent of the cosmic “big bang” and one of Europe's most successful high technology projects.