this is wat i figured out (not necessary correct):

this DG20 is more or less like the previous OPTIMUM plan (RM20)

the few differences are:
1. DG20 pays 20 sen per sec to other service provider
OPTIMUM pays 30 sen per sec

2. DG20 charged by 30 sec block
OPTIMUM charged by 12 sec block (this should be the case)
*but digi silently change the OPTIMUM plan to charge by 30 sec, ive contacted their customer service to confirm this. (damn shit digi)
the reason given by them was to encourage user to use the DG20 plan rather than the OPTIMUM plan. what?? they are punishing their early subscriber/supporter, i hear u say? YES!

unconfirm
3. DG20 cannot use SUPPLEMENTAL (RM10) plan
OPTIMUM lets ur subscribe up to 3 (RM10 or other) supplemental plans

welll........ DAMN this digi. still using it though.

conclusion: sux DG20 plan.