I was having this issue and finally got it to work with the help of DrayTek Au support (thanks Erick!). I’ve got a Draytek 2710vn.

Not entirely sure what did it – installing a new beta firmware didn’t help initially at least (

My current settings are (for others with the same issue, some of these are probably unnecessary):

Remote Access Control – all enabled
PPP general setup – default
IPSEC general setup – enter a secret, only 3des enabled all others disabled
no IPSEC peers setup
remote dial in user:
enabled, 0 idle timeout
PPTP (as backup) & L2TP, IPSEC policy set to MUST
i’ve also set the IKE auth and IPSEC sec settings in the dial in user – the way to enable fiddling with those is to temporarily enable “Specify Remote Node” then edit the IKE pre-shared key to my secret and the IPSec security to 3des only, clicking OK to save those settings then going back in and disabling “Specify Remote Node” and clicking OK again. That might all be black magic…

On the iphone 4.2.1:
server duh
send all traffic yes
RSA secure ID no

Hope this helps – good luck to any others in this situation. Will also post in DSL section.