CCIE Collaboration here. as @Bruce & @DarrenOC already mentioned. The TFTP has to be set to the CUCM servers which are having the tftp service enabled. The quickiest way would be to check the configuration of a working phone. The only reason to set it to the VoiceGW is, when you are running a callmanger express (NOT SRST) there. Please also check your firewall settings. The tftp protocol is not really used anymore. The phones will get the correct servers via DHCP option 150 and then they will download the configuration & firmware files over HTTP/S over the Ports 6970/6972. If these are blocked, then the phones won't be able to download the correct settings
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