Fax sending failed particularly with rightfax

We have WC7428 and WC7835 in our fleet. While sending fax from WC7835 to a client(Right fax) the fax job getting paused and after several retries, it is failed. I tried different starting rate (33.6,14.4,9600) for sending

Same fax job we tried with WC7428 and it works fine.

Any suggestions.