How can you change your cell phone number when sending a text?

for example, somebody sent me a text saying 666 and the number that sent it was 1000000000 so im guessing they changed there number or something. someone also keeps calling me from a blocked number and i called AT%26amp;T and got them to tell me the number and they asked when was the last time they called so i looked on my phone and the last time they called was at 11:07pm and the guy looked to see who called at 11:07pm and it said that i called myself. so i told him a bunch of other times that they called and when he looked it up it said i called myself so i think that they are using a computer with some kind of software to do this. but if anyone could give me a few opinions i would greatly appreciate it.How can you change your cell phone number when sending a text?
Be interest for your work and be happy what is happing to your cell phone ok.