“play-services connected device” - a regular android phone, if you still have one, without modification from the manufacturer (“stock rom”) and connected to a google account is the easiest way to get paid apps onto a /e/ phone, as there is no way to buy apps from the play store, see this thread → Can't buy apps from Google Store
as for the other question in your top post
Besides, is there a shop, or an association, close to Paris where I could speak of these issues with a “physical” expert?
you could seek out the fairphone community, I guess most users that attend the meetings have knowledge on customroms as /e/ and Lineage and as you’re interested in a FP3 anyway - https://forum.fairphone.com/c/participate/local/42 … the forum software allows to search within the category, so Paris has some hits
As for VoIP / SIP, I’d really like to see an in-depth review/comparison on available Apps, with a query I mostly hit search engine spam (“top 10 x”). I did post a small guide to how to use the built-in client, but it’s not the most featureful: [HOWTO] use the builtin Android SIP client to do VoIP