Text messages don't always work right away after porting your number from another carrier. Have you tried turning off the phone and removing the SIM card for a few minutes. Reassemble and reboot the phone. This forces the SIM card to reestablish the connection to the network and the features assigned to your line.
If is still doesn't work, call Customer Service from another phone and have them reset the text message feature on your account from their end.
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Welcome to our family LynetteD . I'm so happy you with us and I know how important text are so no worries because we are here to help. The information Ann154 provided is awesome and also correct. Porting can take some time to go through the process completely and at times may prevent the phone from having use of all it's capabilities but not for long! Here are some additional steps to help with with your issues. Make sure you have both phones turned off completely and then only turn on your new phone. This is simply soft reset that can help resolved minor issues. Are you able to send text, access the internet and make/receive calls? If you continue to have issues, we will definitely need to speak with you on an alternate phone for trouble-shooting. The underscore is also an issue we can address because I'm sure its very frustrating to have this on every text. Lets try updating the predictive text messages Manu>Setting>Language and Input>hitting the gear icon next to the Samsung keyboard and sliding the bar to off. You can always turn it back on. Are you using a text messaging application? Please keep us posted.