Can I use an External Mic with Widex Hearing Aids while Streaming Audio and to Connect Calls?

I have bluetooth-enabled Widex hearing aids that support one-way streaming: audio from the iPhone to the hearing aids only. (Widex music streaming is quite good, by the way!) When making a phone call, however, I can hear the other party directly through the hearing aids, but I have to hold the iPhone to my mouth for them to hear me, because this arrangement uses the iPhone's microphone.

I really need to connect an external lapel microphone for hands-free calls, while walking, for example. I'd prefer a bluetooth mic, but the wireless mics I've found are expensive Widex 2"x2" bricks, inconvenient 2 piece systems (a transmitter and a receiver that plugs into the iPhone), or expensive (over $200) Bluetooth mics. I am willing accept a microphone that plugs into the lightning port with a short, 3' wire.

In any case, my questions are: If I'm streaming output to the hearing aids, can I also use an external mic of any sort for input? Does anyone have experience with external mics for phone calls, and which mics work for this?

Thanks!

[Re-Titled By Moderator]