first of all - do not touch firmware before excluding all other possible causes
for malfunction or You got the SIP-incompatible version 3.63!
compatible 3.99d version image is attached as *.tgz to this thread
http://www.ip-phone-forum.de/showthread.php?t=142690
klick the attachment symbol of it in the threadlist to download.
you need the usb-driver (maybe You need to change usb pid in the driver *.inf file)
and the version 3.33 pc-utility from the
pirelli 3.99d firmware package from siparladi.com, docs are included.
do not use firmware included in the siparladi package,
it may be incompatible to TC300 and could kill it!
pls search an read the other english threads and posts here, user c0ke e.g.
older versions are not available or not recommended.
do not use packages from p2p-downloads or non-provider websites,
they may contain logic bombs.