RCM u/s 9(3)

Reverse charge is a mechanism where the recipient of the goods and/or services is liable to pay GST instead of the supplier.
So advocate is service provider and legal services covered under RCM. Service recipient will deposit RCM.

As per GST rule (sec 9(3)) advocates service is under RCM. So, He don't want pay tax on his outward supplies.

As per notification no. 5/2017 – Central Tax dated 19th June, 2017, Government has specified that the persons who are only engaged in making supplies of taxable goods or services or both, the total tax on which is liable to be paid on reverse charge basis by the recipient of such goods or services or both under sub-section (3) of section 9 of the said Act as the category of persons exempted from obtaining registration under the aforesaid Act

And the being notify by CBEC vide Notification 13/2017 CT(R), the advocate services are under RCM