That will be pain in the ass process, below is my recent experience.
on 16 call maxis asking them how much i need to pay so i can port out my line, as they reply rm 258 due to phone contract, agree than pay - > fail to pork out, cause now they said i have out standing credit.
0n 18 -> settle that balance 400 (i paid extra), than port out, fail again, they said i have 2 sub line under my account, in fact i have no sub line, call them again and ask they said i change my sub line to this account last month (which i never), they said they will solve it for me.
20-03 port out fail again -> call cs -> they said no problem, should be able to port out.
22-03 port out fail again -> call cs -> samething
24-03 port out fail again -> call cs -> they said i have two subline under my account, ask them when they said i change in dec 2015 (last couple day said i change in feb 2016), they said solve for me.
25-03 port out fail again -> call CS and FARK (11am) them until their manager call back -> he told me is sub line issue but they can't help cause tech support is off duty and they don't work on weekend, so have to be next monday only can solve.
take me nearly ten day all payment paid but still can't pork out.
If you had sublines before, even though terminated they are still tied to your account, have to contact them to remove.
Had the same issue with the port out fails before, I visited their service center to sort out.