Different type of construction - older handsets used coils and iron core dynamic speakers within the receiver. This made for heavy and large handsets that required a reasonable voltage to shift. ~ Modern landline handsets tend to use a piezo-ceramic speaker that needs far less power and is lighter to build into a device.
Also looking at your loss, I’m guessing that the OPN is a bit shy on power/coupling into your ear especially in the lower pitches. Your audiologist should be able to provide you with an enhanced program for phone use.
The slightly unusual shape of your loss also may indicate LF dead-spots in the basilar membrane - that aren’t easily identified due to nearby overlapping frequencies. The issue with the phone accentuates the problem as the bandwidth compresses due to the way telephones work. In your case, low pitch sounds that would sound OK one to one are likely compressed to a slightly higher, narrower band, if this band sits on a dead-spot around 500-750z then it may still be difficult to hear even when amplified. This would be true of the telecoil also, as it operates over the same bandwidth as the phone.
Your mobile phone operates on a different technology platform that isn’t constrained in the same way.