The problem of restarting devices during call is an issue with some touchscreens. It may be solved by up- or downgrade to not affected Firmware versions if you perform a factory reset in recovery after that. If the latest version don't work for you try out flashing older versions where you don't had this issue.
If that really don't help you can exchange your device over the seller.
If your seller really don't help you please contact firstname.lastname@example.org
. Describe your issue exactly, refer to this post, and provide all informations about your buy/device (date, order number, seller, price, color, IMEI, communication with seller, ...)