I have 2 Greenlee ones (£50+ for used one)
1 is branded "BT" & beeps when their is ADSL on the line (you don't really need this)
there isn't much more too them than than using a cheap one piece phone,
if you do want one- this is cheap & still in the packet
https://www.ebay.co.uk/itm/GREENLEE-DSP-COMPACT-TELEPHONE-TEST-BUTT-SET-M0452-00GA-NEW/254245502757?hash=item3b32367f25:g:Gq0AAOSwZRZc7CB0&frcectupt=true