It seems that the issues that are reported in other comments have been fixed :
- the country and zoiper icons can be disabled (there is an option for each icon)
- only numbers that 'look like' phone numbers are taken into account
- I didn't get any http error
- you can disable the plugin for a page / an entire website

The only point that might be enhanced : it would be nice to have on option in order to take into account only tel: hyperlinks

Highlights unrelated numbers such as postal codes, ISBNs, VAT IDs as phone numers. That really needs to be customizable. Puts its icon and a county flag next to each number which makes the affected web site look weird. Not usable like this. Uninstalled after 5 minutes.

Great add-on, does whatever it is supposed to!

I get an error that it can't find the file association for zoiper.
How do I add this to firefox?

https://ibb.co/kamGC9

Nice if it identifies a number as a telephone number, but it identifies almost all displayed numbers as being telephone numbers and many telephone numbers not.
And it always displays a Zoiper Logo in fornt of each number which seems to be a telephone number.
No way to exclude certain pages (for example our back end of our online shop gets filled with zoiper logos and ech order nummer is a telephone number....)
So a lot of work to do, I would say almost useless like this, or even iritating.

Does not work properly with https sites. introduces mixed-content errors.