This caller left a text message, saying that he (she) had not heard from me in a long time. I don't know anyone at this number.

Same thing. When i asked who this was she said you have to add me on yahoo messenger to find out and gave her yahoo id.

they texted me sayin that theyhavent heardfrom me in a long time. i asked who thiswas and they told me that if i want to know i had to add them to yahoo messanger. I did and still didnt know who was callin. i do now thanks to this web site. thanks

The same thing happened to me too. Did it say word for word
"hi <insert first name here> u there? Have not heard from u in a while."

I responded

"who is this phone book got wiped a while back"

Then this person said

"lol if u want to know who this is you got to add me on Yahoo messenger. my name is textiewoman85."

Capitalization and everything right?

I got the same text and  Yes I added the number to yahoo, no response from the person after that.
I'm sure that some one is just trying To get your personal Information