I have a simple phone keyboard and prefer to use Net10's SMS page:
and I've used it many times for my wife, son, and to test my own phone - all net10. Some time in December, it has stopped working.
If this url works for you, I'd like to know.
If you know of another URL - please let me know.
Then I did it with a Tracfone number (even though it specifies to use a Net10 number). Same thing. Appeared to work but the msg never came through.
Then I used a SafeLink number and got the error 'Tracfone does not recognize the number' or something like that.
It would be interesting to see if you get the error msg when inputting a StraightTalk number. (I don't have access to a StraightTalk number.)
Just to check, I sent a msg from the ATT phone to the Verizon phone (the regular way), came through immediately.
I did not log in to my Net10 account for any of the tests. Both Net10 phones are regular PayGo, not Unlimited.
Not a direct answer to your question, but Google Voice might be a good solution for you.
I just tried too. Never received message.
Didn't even know this service existed. It would have been AWESOME to know about. However, yea, it doesn't seem to work now. This is pretty neat, but Ionno how much I would have used it. Like that other guy said, google talk would be a good solution or you can do it form skype. Still, neat feature. They should bring it back!