Hi Rock23-
welcome to the Apple (user-to-user) Community!
Re: "Can iphone be set to simultaneously ring through hearing aids AND external speaker?"
Am no expert in this area: Yet it seems you would have to choose one or the other, as you adjust the audio while on a call: See: Apple Support's online article: While on a call on iPhone - Apple Support
Eg: says: "When you’re on a phone call on iPhone,
you can switch the audio to the speaker or a Bluetooth® device".
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Re: "because I don’t have the hearing aids on I don’t know someone called
You may need to check you do not have "Silent mode" on by accident:
How to put your iPhone on vibrate, ring, or silent mode - Apple Support
Select ringtones and vibrations on iPhone - Apple Support
Else: Might it help to have an extra loud ring-tone on your iPhone ?
You might want to change the ring-tone to something you can hear without aids.
🛎 🎺 📣 🎶 🚒 🥁
Apple support have an article online that may help:
Change iPhone sounds and vibrations - Apple Support
or
(In the "Contacts" app you can choose particular ringtones for certain callers if you wish)
Use tones and ringtones with your iPhone or iPad - Apple Support
You can even check out other ring-tones available: eg: iTunes Store
Get ringtones, text tones, and alert tones in the iTunes Store on iPhone - Apple Support
or you could search online for extra-loud sound effects and loud ringtones if need be.
(ringtones can be up to 30 seconds long)
All the best :-)