How can I make the phone automatically disable mobile connection when calling?
You may want to talk to your carrier about your phone or service because data is separate from  voice (if you are not on a CDMA network). Voice is using GSM/UMTS and data is HSPA. Your data network usage should never interrupt your voice call, even if you use CDMA. The voice should take priority and stop the data usage to make the call. There is either an issue with the network you are on, or the phone.