Are you using the earphone socket on the phone and if so do you have the phone volume at max?
At the risk of teaching Granny etc, volume controls are designed to reduce volume rather than increase them. if using 2 devices in line it is good practice to run one wide open and use the other as control.
If you do have the phone at max and it still isn't loud enough the only suggestion I can make would be to introduce a pocket size headphone amp into the chain to add gain but I'll be surprised if you find it's needed.
Out put by bluetooth is a different kettle of fish....
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