Hi there Cindykc,
Hmm, it's not clear whether you have Missed Call Service or Voice to MMS active on your service.
Either way you should be able to hear messages left longer than 10 seconds in duration.
Is it happening for just the one caller or multiple?
What happens after 10 seconds? It just hangs up?
Ultimately you'll need to contact Optus technical support. They may just need to re-provision/tweak whatever missed call feature you're using.
Text "OFF" to 159 to turn off Missed Call Service
Once deactivated, dial free call 1509 and follow the prompts to turn Voicemail back on.
Okay something does sound off with your service then, you will need to contact Optus directly to get to the bottom of it.
Let us know how you go.