If you are sending calls across the WAN to make local and LD calls, you would need to put an analog line into one of the UC520 FXO ports and create POTS dial peers pointed to the FXO port for calling. Make the VoIP dial-peers Preference 1 and the POTS dial peers Preference 2 or above. That way of the system can't do VoIP calls it will hit a 'lower' preference dial-peer.
Essentially, SRST works like the router is a CME/UC520 standalone system. That's why UC520 doesn't support SRST, it's already essentially running SRST.