Top up when not using phone

Hi, I live in Australia but was travelling to Thailand regularly so purchased a DTAC sim.

I always top up the sim so I don't lose my Thai number. My questions are:-


1)  Do I need to keep doing this to keep my Thai phone number.....I need this number for business when in Thailand?


2)  I just lost about 800 thb in credit due to not travelling to Thailand for the last 12 months.  Previously I was able to purchase approx 200 thb with an expiry of 6 months but when topping up yesterday,  I spent 200 thb and it will only last 1 month.


3)  How can I make my credit last longer when I am not using the sim?


Any advice would be much appreciated.


Thank you.... Diana