Well, I've checked it again last night, and I really did miss something small in the configuration.
I've used "ppp authentication ... optional callin", whereas I should have used "ppp authentication ... callin".
The configuration guide I've followed to setup my router supersets the 12.3 IOS "L2TP Client-Initiated Tunneling" guide, which you've graciously provided. The guide is a generic Client-Initiated Dial-In VPDN Tunneling for the 12.4T IOS, and is available here:
»www.cisco.com/en/US/products/ps6 ··· b6d.html
I have another question. Dialer interfaces can be configured to be persistent, but the persistent setting cannot be applied to a virtual-ppp interface.
Does anybody know how can I make a Virtual-PPP interface persistent?