Adding some additional information to my prior post:
It sounds like the EJ-10 boxes that are labeled as coming from the "IWE" company (Industry West Electronics) definitely will not suffice to send a signal over a phone line. Their design only supports receiving a signal. Also, for a period of time after Emtech procured IWE, the EJ-10 was sold with the same design but had an Emtech label on it. The tech support guy from Emtech that I spoke with today said that EJ-10 devices that were sold up until about 2014 or 2015 retained that older design, so they also would not be suitable for sending a signal over a phone line.
There should be an Emtech sticker on the back of the box (where all the cables are strapped on) that has a 4 digit date: MMYY. If the year is before 2015, I would be suspect of that box.
The tech support guy said that after 2015, the EJ-10 box design was changed and that it should suffice for sending over a phone line as well as receiving. He also said that the EJ-R box would provide an even stronger signal. I didn't ask him about that, but I thought it was an interesting statement.
The Help pages for Audio backup on lds.org (https://www.lds.org/help/support/webcas ... s?lang=eng
) specifically refer to using an EJ-R for the sending side and an EJ-10 on the receiving side. I have requested an EJ-R from our FM group, and once we have one, I will run some tests and let you know what we find.
I hope this helps!