As in the photo, BT Stratus 1500 cordless answerphone plus additional cordless handsets; two more not in this picture so four in total. No longer needed as we've been switched to digital phone connections as part of a broadband upgrade.
Sorry but, unusually for me, I can't find the User Guide though I do have a digital (PDF) copy available to email if you can't read it online (and I'll keep looking!). But it is pretty much "plug and go".
Working fine though the batteries have been installed for some time so don't last too long.
The manual can also be found on the BT web site, see:
https://www.bt.com/content/dam/bt/help/legacy-ug/phones/BT_Stratus_1500_use...
Last chance before it goes off for "recycling".

BT Stratus 1500 Answerphone with 4 cordless handsets (Chalfont St Giles HP8)