I am not sure which alterations have been made by the vendor (AbovePhone).
Yes, I have been looking into Above's knowledge base (Wiki, videos).
The problem seems to be that there are possibly three accounts, the device itself, a WebDAV sync account, and in our case Telegram. Then there is also the SIM card where addresses can be stored. it seems not all of them are accessed by the contacts app, or not all of them are revealed to the telephone app.
This is quite confusing.
If there would be some tutorial which explains which contacts in which "account" are accessed and shared with other applications, that would be really helpful.