There are a two ways you may retrieve messages from your old voicemail inbox.
One way would be to look in your Carrier's Voicemail App. Most carriers provide a way to view voicemails left in their system through their app. If you have access to this, your old voicemails will still be there.
If you do not have access to their app, then simply dial your old voicemail access number from your cell phone. If you do not remember your access number, then you will need to contact your carrier and ask for this number.
If a carrier requires you to reconnect to their voicemail system in order to listen to your old messages, make sure to reactivate your call forwarding to YouMail so you can continue to use our services. Check out some articles here on how to reconnect to YouMail.