Not sure about the FDCPA but under California law you have that right to request communications in writing only:
If you prefer that the agency contact you only by mail, you may ask them to do that. We suggest that you make that request by certified mail and keep a copy for your records (15 United States Code sections 1692c(1), 1692c(c) & 1692d(5); California Civil Code section 1788.11(d) & (e)).
So they have violated California statutes for certain.
Not under CA law but it also references US Code 15 1692c(1), 1692c(c) and 1692d(5)
I didn't find it there either. I do know if you send a cease and desist to a CA they can call you and let you know they are either going to stop collecting or sue. This isn't the same thing. You are only asking they contact you by mail. I would see no reason for them to sue.