I am actually at this moment trying to get a branded deal going, for a large global hotel chain and also looking at doing it for my own service.
Verizon and others do this with a Yealink. The main reason is when you sell to a business and not to IT, they've never heard of Yealink. There's no need to give a customer the desire to google Yealink and then learn about FreePBX etc. Yealink doesn't have the brand equity Polycom or Cisco does.
I was really surprised when Sangoma claimed to not be using Yealink for the design. It's a clear rip off of the t4 series design but I did hear early on they sourced Texas Instruments for boards and some chipsets that have wideband audio issues.