Register SIP trunk for every call

Hi all,

I´m new to the FreePBX world and I´m setting up a SIP Trunk with But I want to still use the SIP account from a SIP phone not connected to the PBX outside my office.

Is there a way to make the SIP trunk register each time a call is made through that trunk and unregister when the call ends?

This is so that the account is not blocked to use when nobody is calling at the office.