text messaging help please

I currently have a Sony Ericsson Z500a cell phone and I have problems with inter-carrier text messaging. Nothing at all is wrong with cingular, but with verizon (which most of my friends have), they usually receive my text messages immediately and one of three things happens when they reply... The first thing is they come immediately, this is very rare. The second thing is receiving them later... much later, usually 12-24 hours later, this happens some of the time. The third and final thing which happens is that I don't receive the messages at all, this is what happens most of the time and I think my phone is to blame because cingular has agreements for text messaging with these other carriers. Could you please help me fix my text messaging problem? Also, if you don't know the answer to the first question, are there any cingular phones that anyone here owns or knows that work well with inter-carrier text messaging? If not then I may just switch to verizon. Thanks for the help