I think the reply you suggested is also not correct.

It should be:

PortOut YES 6012345678

Notice the alphabet E & S are in CAPITAL besides P and O

Even slight detail like that might prevent you from getting ported out.

When the port out SMS comes in, I would just highlight the entire line the copy & paste to be safe.

Not all telcos are as tedious as this.

If you're porting out from YES or Unifi Mobile, you only need to reply YES back to the telco.

TuneTalk is even simpler just reply Y

Safest is just copy EXACTLY to what they request in the first port out SMS that comes in.

Just follow exactly what they request don't take my words. MATCH EXACTLY what is put out note the spacing, capital as shown. It differs by the telco you're currently porting out from. Some make it relatively easy some make it hard to discourage you by intentionally tricking you to miss out a single mistake of capitalization/spacing.