Good Morning

I wonder if anyone can help. We have quite an old phone system (Samsung DS-5014S) We have 6 lines coming into the building (all use the same number). What I want to know is does this phone have a feature which allows someone to answer an incoming call and then one of my colleagues in another office can listen in to the same call. I have read the user guide but I couldn't find this feature. If anyone can help, I would be appreciative

Yes, the feature you want is called barge in or intrude.
It is not setup by default and depending on the model of phone system will depend on how you set this up.
You have listed the model of the phone, the system itself will be a box on a wall somewhere that will say something like samsung dcs24 or samsung idcs100
You will need a tech to set this up as it's too complicated to assist here