I wonder however if it`s possible that they could be based on the same reference design though…
I googled a little and it looks like the same thing that exist for video cards, ie having many video cards from many manufacturers based on the same reference design from NVidia and AMD exists for SIP phones as well…
If they are based on the same reference design from a chip manufacturer they could exhibit similarities without actually being built by Yealink…
Have a nice day!
Nick
PS: It might also explains why so many manufacturers exhibit the same SIP quirks/issues because the firmware can actually be part of that reference design…