Reset your UC520 back to factory default - you should be able to call back and forth between the IP phones and analog phones (do not need to add a dial peer etc). The analog phones are extensions 301 to 304 depending on which port you plug into.
On why it does not work now - see if you can try using an destination-pattern on the POTS dial peer of 321 to 324 (instead of 2xx or 301 - 304) - it maybe that you have multiple options to route calls to specific extensions (can confirm after seeing your config).
