I'm able to send text messages out and people can receive them.
When some people text me back, I never receive the texts.
This just happened when I upgraded to the HTC M8.
Is there any fix for this or will I need to return my HTC and get a different phone?
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Upgraded to the HTC M8 from which phone. If it was from an iPhone, I would suspect the people you are not receiving the texts from are iPhone users and they are unsuccessfully attempting to iMessage you.
If this is the case, text the word "stop" to 48369 and you should receive a reply stating you are unsubscribed. As long as your friends have the option of sending as an SMS when iMessage is unavailable checked, you should start receiving those texts.
A power cycle is always good to perform when issues arise to perform press the upper volume and the power button for about 20 to 25 seconds you will see a count down banner on the screen showing how long to hold as well..b33