I'm using prepaid service, and I've read on some forums that the problem might be caused by the sms the provider sends when the call is disconnected, informing of the cost of the call and your balance.
In Portugal Vodafone don't allow to take off that message, so the solution is to return your Windows Phone and get an Android phone (I've been told this 5 times in different stores.)
Post By NOKIA.......
Good news! We have brought forward the software update schedule for our Nokia Lumia 710, and have begun rolling out the latest update 1600.3015.8107.12070 to India and Singapore. We will add more countries and variants to the rollout based on market requests.
The new Lumia 710 update brings the following enhancements: • Smoother device performance (when ending a call) as well as
• other minor performance and usability improvements