What constitutes a sub reporting?

[LocDog249]

Had a pretty good BV game last night, in a fairly loud gym. My question is, what constitutes a sub as giving their number to the score keeper? The keeper said the visiting players were not verbally giving their number when they reported, and that they were only walking up and pulling out (not untucking) the front of their jersey to show the number.
To me, if the score keeper sees this, then he should also see the number and realize who is reporting in. I know most score keepers have no problem with this as "reporting" but it did get me thinking about what is considered legally reporting.

(My first post!)