You have a couple of options regarding that….
You can use EC 500 which will ring the phone and extension, just be sure to understand the amount of trunks that you have available because each call uses 2 trunks.
You would need to goto
Change off-pbx-telephone station-mapping (enter the station number)
Change off-pbx-telephone configuration-set
Change station (XXXX) and add and EC 500 button to the phone
Or
You could manage this with coverage paths and coverage remote, but EC 500 would be the best option in my opinion.
Try this link for further details
1. EC500 Extension to Cellular Release 4 Installation and ...