Oh, that's frustrating. Unfortunately, I don't think you're going to change them. I don't have a cell phone at all (not in my budget), so I feel your pain.
I think the easiest thing to do would be find a way to work around it. For example, I have a landline, and if someone texts my landline (they forget it's not a cell number), the text gets read out loud to my landline by a robot voice. It's kind of neat.
My phone company also provides a free, opt-in service where voicemail messages get emailed to me as a sound file. It's awesome because even though I don't have a cell, I usually have a computer with me when I travel, so my phone messages show up in my email.
Does your phone company provide anything like that?
If not, unfortunately, you'll have to decide between spending more $$ on texts, or missing out on events. It's not fair, and your friends should respect your financial decisions and make the effort to call you, but I'm finding more and more people see a cell phone with a heavy text plan as a "necessity" and won't work around friends who have made different gadget choices.