Samsung X42M - text msgning does not work consistently

I own the phone, but my daughter uses it. I'm posting this for my
daughter. The carrier is Cingular.
The model is a Samsung X42M. I've owned it for about 2-3 years.

When she sends a TextMessage (TM) to some people it is transmitted
correctly. When she sends a TM to others, they dont get a TM at all.
And some get a blank TM. I have not analyzed who it works for and who
it doesn't....

Two days ago she took the phone to a Cingular dealer at a retail
location. The employee said she needed to go to another type of store
where a particular Cingular Techneician is located (maybe he did not
know what to do, I dont know. I was not present). We have not done
that yet.

Any ideas what might cause such irregluar, and inconsistent TM's?

Is it a problem with possibly the carrier of the receiving phone? Is
it a setting on the phone that needs to be changed?

Is the phone possibly broken and malfunctioning? If so, would it be
cheaper to just buy a new one?

Any advice appreciated. Thanks!!!