Let's put a scenario for an easier understanding 

Let's say that you've top up RM10, so it entitles you for 10 days of free calls to other U Mobile numbers from 12am to 6.59 p.m.

Within this 10 days, if you called U Mobile number in between 12am and 6.59 p.m and the duration of call per day is less than or equal to 60 minutes, then the call charge is foc.

However, if you called a U Mobile number in between 12am and 6.59 p.m but the duration of call is > 60 minutes then you will be charged(with normal 9 cent/30 sec, I think).

Meanwhile, if you called a U Mobile number that is outside of the free call time(which is 7PM to 11.59PM), you will be entitled of a cheaper call rate which is 3 cent/30 sec.

Have you reloaded RM100 100days before? If yes then it makes sense.

For the reloads, the number of days of validity doesn't stacks/accumulates. The principle is that the validity will be added from the day you've reloaded it and choose the date whichever is longer(new credit validity date vs old credit validity date).

For example,
If you've reloaded RM 100 on day 1, so your credit will expire on day 300. But, you've reloaded another RM100 on day 100, so the validity of the credit will be added from the day you reloaded(day 100).

new validity date = 100 + 300 = 400 days later
old validity date = 300 days(from the earlier explanation)

Hence, the longer validity date which is the newer one will be chosen as the new validity date. Otherwise, the old validity date will be used.

Is that clear enough? 

p/s: do drop by and tell me if there's any mistakes eh 

can you tell me when did you last reloaded it(disregard the recent reload)?

frequency not same, not support

most likely, no. Unless U Mobile is granted with the 1800mhz spectrum, or Maxis/Celcom is willing to share their infra with U Mobile

If U Mobile were to hop on onto Maxis or Celcom's 4g network then probably you could see something soon. Otherwise, don't even think about it as requesting for the band takes a lot of time and building the infra will take a lot of time as well(Digi took a year+ to implement it)