5201 and 5302 phones are compatible with the 3300 on all software loads I am familar with (ICP 6.0 through MCD 6), but you must be using DHCP to configure the phones with properly configured options 125 or 128+ to get them to work, as they have no displays you cannot manually setup the ICP/TFTP address. I am not positive here, but the 5201 phones might require option 128+ as they are fairly old, I don't have one to test with though.
