I have a basic phone (Samsung with a slide out keyboard) and a basic plan with 700 minutes and unlimited texting. Am now receiving messages that I have too many voice mails stored (full mailbox) when I only have 19. There are some I've saved that I can delete, but can't figure out how to scroll through them. Also, I keep hitting a limit on stored texts and have to keep going in to delete--have about 230 received and 240 sent. I'm not a big texter compared to most, so this doesn't seem like a lot to me. I know it's a dumb question, but are the texts stored "in the cloud" or on the phone?
The voicemail is limited by 2 factors - # of messages and length of messages. If one of messages is particularly long (more than a minute) it would reduce the total space available for the other messages. To delete the messages you no longer need, listen to them. As the message is playing, press 7 to delete and move to the next message. Press 3 to skip through that message and on to the next, and press 9 to save the message and listen to the next message.
Your texts are stored on the phone, and basic phones don't have a lot of storage, so 500 texts sent and received is a lot for a basic phone. Delete what you don't need.