Firstly, the voice path used for a forwarded call has nothing to do with
the fowarded phone (your cell) - once the call is forwarded, the cell
phone is no longer involved in any way.  But, the call processing
equipment for your wireless carrier *is* involved, it's still involved
up to it's eyeballs.  The inbound call is terminated in the wireless
carrier's switch, and the forwarded call is then re-trunked toward the
final terminating switch which handles your landline service.  Your
carrier is at least partly responsible for the voice quality of the
forwarded call, and they'll need to be involved for a resolution.

Call the CSR folks to begin the process, and I'll help you start with
this, the CSR people aren't the most technically competent people for
this type of issue (nor should they be), so help them help you by
offering them your patience but expect to eventually speak to a tier two
technical representative.  Those folks are going to ask you questions
for info they'll need because they'll have to figure out what trunks are
involved for your specific situation.  They may even ask you to make a
few test calls so they can watch the call progress with their equipment
in order to learn a number of variables they'll be dealing with.  Once
they've got these down, they can continue on their own, but again, your
patience and assistance will be key to helping them help you.