Like most rules in 5e, there is ambiguity here.
Casting some Spells requires particular Objects, specified in parentheses in the component entry. A character can use a Component pouch or a Spellcasting Focus (found in “Equipment”) in place of the Components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.
If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell.
This makes it clear that the silver cage requirement of Soul Cage cannot be substituted away with a Spellcasting focus. You must have that silver cage.
A Spellcaster must have a hand free to access a spell’s material components—or to hold a Spellcasting focus—but it can be the same hand that he or she uses to perform somatic Components.
The last paragraph is the most interesting one. The "same hand as somatic" can be dropped.
A Spellcaster must have a hand free to access a spell’s material components—or to hold a Spellcasting focus.
This can be read in two ways. Either
- You must have a hand free for any of "access material components" or "hold a Spellcasting focus" you use while casting a spell. If you want to do both, you need 2 hands free.
- You must either have a hand free to "access material components" or "hold a Spellcasting focus", and satisfying a hand free for either requirement is sufficient.
Which one applies requires you to talk to your DM, who may rely on rulings by various people at WotC or not.
Fluff wise, the second would be doing something like touching the component with your focus (you have to have it, but not hold it).
I'm a Hexblade Warlock with a glaive and the Improved Pact Weapon feature, which makes my summoned glaive my Spellcasting Focus. I wonder if there's a way to kill an enemy with my glaive then use my reaction to cast Soul Cage.
Using a glaive as a weapon requires 2 hands, but using it as a Spellcasting focus only requires 1. So you can attack, kill, drop one hand, grab the silver cage and use it with your now-free hand. The other hand can hold the glaive and use it as a spellcasting focus (assuming you care; some spellcasting focuses provide advantages to spellcasting).
Removing your hand from holding something is not considered an object interaction. Possibly putting your hand back on it might be. The "1 free object interaction" might be taken up by putting your hand back on the Glaive. You should discuss this with your DM.