Why does my dial tone continue after correspondent answers my phone call?

[iMore Question]

Dial tone continues after correspondent answers my phonecall.

This happens frequently and is very annoying - Apple store suggested upgrading to IOS-7, which didn't solve the problem (but made the iPhone very slow).
Neither Apple nor the operator have ever experienced this problem.
iPhone model: 4S 8Gb
operator: BASE
country: Belgium

 

[Bigeric23]

Re: Dial tone continues after correspondent answers my phonecall.

I've never had this issue, either.