Nowadays there's Mobile Number Portability (MNP).
You can change any Telco you wished to, but your numbers remained the same.
For example, I got a 012 number, but its now belonging to Digi, instead of traditionally said Maxis.

So u have tested DiGi via prepaid card, test it with other telcos as well, then once you've decided, u just change the Telco via MNP to maintain the ownership of your number.