I don't have a SPA525G2, but I do use cp-7920, cp-7931, and cp-7961 using SCCP on asterisk.
You can try to login the web gui of the phone and select Admin Login near the top of the page.
If the phone is at factory defaults then only a few changes are needed to register the phone.
Select Ext 1
change the sip port to 5060
enter your pbx ip for the proxy
set register expire to 3600
for user id enter your ext #
for the password enter the extensions password
submit all changes
not sure if a reboot is needed