While talking the phone is switching off.why?

Unless you're Verizon phone has been unlocked. If it isn't, then it's carrier locked and won't work until it's unlocked.

Make Sure your Phone is asking for unlock code with a AT&T Sim to unlock this if yes then apply here to get the unlock code.
unlockninja.com

you need to contact straight talk's support and tell them your mobile number porting from verizon to straight talk has not been done properly and verizon is being contacted instead of straight talk phone. Verizon will have to release your #
but you want Straight talk to port your # to them (it could get lost if this doesn't happen )

you will have to have it flashed because straight talk will not activate s4 from Verizon I have tried myself bought the kit and wasted money. The kits are no refundable

Switch the phone off and remove the battery. Leave the battery out for about 5 minutes then replace and switch the phone back on. leave the phone switched on for about another five minutes before attempting to text again. This cured the same problem on the same phone for me.

Does your phone contain a SIM card?

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14. Push to talk Push to talk (PTT) is a two-way radio service available over a GPRS cellular network (network service). You can use PTT to have a conversation with one person or with a group of people (channel) having compatible phones. While you are connected to the PTT service, you can use the other functions of the phone. To check availability, costs, additional features and to subscribe to the service, contact your service provider. Roaming services may be more limited than for normal calls. This feature may not be available, depending on your phone. For more information about signing up for the PTT services and the usage, contact your service provider. To connect to the PTT service, select Menu > Push to talk > Switch PTT on. LOOKS LIKE YOU HAVE TO SUBSCRIBE TO USE IT! hope this helps.

battery spoiled. Replace battery

whenever you switch lines, if you let the original person your talking to know to disconnect the line the call should end. When you switch lines the phone won't automatically hang up because you may want to switch back to the original call, but if the person your talking to disconnects that should take care of your issue.