You can use a smartphone app like Wave Lite or add the mobile number to a ring group using #0000000000 and the call will also ring on the mobile number next to the other extensions entered.
Years ago, I recall reading about Bluetooth base stations that would establish a SIP channel to the PBX, and automatically send inbound mobile calls to the PBX whenever the mobile is in range of the BT base. I don’t know if such a thing still exists or not.
From the answers posted here it seems like not everyone is clear what OP is looking for.
To me it seems like OP wants calls to his cellphone to get delivered to the PBX and the PBX should handle the call routing.
The question is what is your end goal?
A) Did your business grow and you want to use your cellphone number as the official phone number? Then you can port your cellphone number to your provider and you will get a new cellphone number.
B) If it’s something else, you might want to do a simple call forward to any free DID on your PBX.