Mobile Data issue when making phone calls

[MrsSmith410]

Hi..I've only just gotten my Xperia Z so I'm still getting used to it, but I've noticed that when I make or receive phone calls the Mobile Data screen opens..I'm not sure what I need to turn off/on or what settings I need to change to make that not happen.  Currently my Mobile Data is set to "ON"...

Thanks

[Uliwooly]

could you elaborate?

also if you are not using data then 

settings > more > mobile networks > set it to GSM only