Your free calls validity will follow the maximum validity of your account.
For example, if you top up RM30 on Day 1, you will have free calls until Day 30. If you make another top up of RM10 on day 15, your free calls validity will still be until Day 30 and not Day 45. However, if you make a RM30 top-up on Day 1 and make another top-up of RM10 on Day 25, then your free calls validity will be extended until Day 35.