I think it's this: Yealink Battery W56

No, it's not a standard battery (reference W56-BATT or article no. 330000000000). You have to google "yealink w56h battery".
There are different sellers, in Switzerland and in Europe (I don't want to advertise any of the sellers).
The difficulty will be to find who has stock...

I think these are just normal rechargeable batteries that you can buy at any discount store!