P000 controls the lock access if you are using the Siemens handheld programmer to access a Probe LU.
P799 controls the access if you are using a HART master, like PDM software.
A remote HART master can still change configuration, if P799 is set to allow this.
The communications control parameter P799 must be set to 1 to be able to write
parameters to SITRANS Probe LU. 1 is the default setting for P799. (pg 77 pdf)
If P799 is still at the default value 1, then you can access the Probe LU with PDM software and a HART modem.
Note that the default unlock code is 1954. If P799 balks at other values, try 1954.
I've heard that there is a backdoor password for P000 that can be used with the IR handheld programmer, but that password has to given out through a formal Siemens channel, hence the statement to contact Siemens.