HP has issued an Advisory for the iPAQ hw6500. When the auto-correction feature is used on SMS texts, the SMS available characters count drops dramatically. This issue occurs because auto-correction uses the Unicode text scheme, which consumes more bytes per character (a ratio of approximately two-to-one) than other text schemes. When you create SMS text using Unicode, your text will be limited to 70 characters, as opposed to 160 characters with other schemes.
You can find detailed instructions on the fix by following the ‘More information’ link. Essentially, HP advises to eitehr disable the auto-correct feature or Unicode support.