Yealink T42S and T42G Phone stuck at Welcome

[Pierre06]

Good Morning and Happy new Year !

I've seen there is a similar topic, and decided to make a new one, because i posted from month but get no answer.

In my company we have some 5000 Yealink Phones (T42G, T42S, T49, T19,...), most of the phones are T42S and T42G.
And a non neglectable number of T42S and T42G sometimes remains stuck at Welcome screen.

I've tried to restore factory settings, but not worked, phones still stucked.
What is the procedure to recover the use of these phones ?

I hope i'll get some tips to solve that.
Thanks by advance,

Pierre