Wireless phone reset

The question was... are you connected directly, wirelessly, to the Freebox?
And what does the screen display (icons and others)?

If so... have you disconnected the handset from its original base, so that there is no conflict between the two bases: the new one (the Freebox) and the old one (Philips) which is only serving as a charger now... (See page 33 of the manual)

Note: When connected to a box, directly in DECT, (wirelessly) the functionalities of the phone, as described in its manual, may undergo significant changes.

connected to the box via ethernet cable by wire.
The connection cable has RJ11 connectors that look like those of an Ethernet cable but it is not an Ethernet cable.

But then why do you say in message 2: "When activating detect freebox"? Where did you find this procedure?
What type of Freebox?

Once again, what does the screen of the handset display (icons and others)?

Also check that the keypad is not simply locked.
A long press on the * (star) key (at the bottom left of the zero) locks or unlocks the keypad.

The handset is now associated with two bases...
The Philips base and the Freebox Revolution, and as a result it is unsure which one to connect to.
Unplug the cable going to the Free Revolution, turn off the power to the Philips base, and perform the procedure to dissociate the phone from the box
https://www.free.fr/assistance/2503.html#step-2506
Once the dissociation is done, turn the Philips base back on.
Once the handset no longer has to manage two bases at the same time, everything should work better.

You can also check by simply turning off the power to the Philips base to see if the handset behaves normally.
If it does, and if you prefer the wireless solution (handset associated with the box), then you need to perform the unsubscription procedure for the Philips base (Manual page 33).