when running as a push gateway, will it register periodically to backend sip server?

I just read the document here: https://wiki.linphone.org/xwiki/wiki/public/view/Flexisip/Push%20Gateway/

As we all know the sip client (such as Linphone) won't run in the background (that's the reason we need push gateway) to maintain periodical registers. What will happen when register expires? Typical backend sip server won't route INVITE requests to an offline extension. Will it register periodically to the backend server to keep online?

Appreciate any explanation.