If you looking to have more them 100 messages
if you have the new layout under the pbx
Click on Voicemail Admin
Click on Setting
Under maxmsg change as how many message you want.. 100 is by default can go up to 9999
Then click submit at the button
if you don't have the new layout under pbx.
Click one Unembedded IssabelPBX under pbx
Go to setting and click on voicemail admin
Click on setting and make the same change under maxmsg.