Q1. Which one is correct?
1. All the phones are outdated.
2. All phones are outdated.
3. All of the phones are outdated.

Q2. Is the word "Signage" a countable noun or uncountable noun?

Function: noun
Date: 1976
: signs (as of identification, warning, or direction) or a system of such signs

Q3. Which one is better?
1. All the signage is outdated.
2. All the signage are outdated.
3. All signage is outdated.
4. All signage are outdated.
Q1 All are correct. Note that 2 Means that every phone in existence is outdated.
Q2 Uncountable
Q3 'All the signage is outdated' is correct. 'All signage is outdated' would mean all signage in existence anywhere is outdated.
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Thanks Mike!
Always a pleasure Emotion: smile