I understand that you’re not talking about streaming (sound coming from the iPhone), but you’re talking about sound coming from the speakers inside your headphones.
Perhaps you’re not familiar with how the Tcoil in hearing aids work. What you’re doing in your case sounds like you just let the speakers inside your headphones generate sound waves through the air, reaching and picked up by the mics of your hearing aids. That’s the conventional way of listening, but not the tcoil way. It can cause feedback because the headphones cover up the ears and hearing aids, creating a feedback loop.
But what you don’t realize is that the speakers inside your headphones, beside pushing out pressures into the air to create soundwaves picked up by your HAs’ mics, the magnets that’s part of the speakers inside your headphones also generate the equivalent electromagnetic waves in parallel to the sound waves in the air. So in full-timeTcoil mode, the HA uses only the Tcoil to pick up the electromagnetic waves and not use the mics to pick up the sound waves. This solves the feedback issue caused by the headphones covering up your ears and HAs.
Apparently, @a2wheelerte said that the Oticon OPN S1 also has a mixed mode that uses both the tcoil and the mics together so you can hear sound from the electromagnetic waves from the headphones AND still pick up surrounding environmental sounds using the mics (in case your headphones are the open-back or on-the-ear type that lets in external sounds).