Gizmo is a voice over IP application, comparable to Skype, but embracing open standards. Learn more about Gizmo, create an account, and get a client here.
To voice post to your journal, just call ljvoicepost from your Gizmo client. It should work exactly like the normal voice post service does – just use the Gizmo dialpad.
Update: We've found a potential issue for people with slower connections - if you're having problems calling in with Gizmo, check this link.