First. just make JTAC key a part of the mortar kit/Add JTAC channel into OT’s headset.
Second. Radio Telephone Backpack exists, and alamo, and the ground command tent with a phone in it. The Ordnance Technician’s workshop literally has a phone in it. If OT does not pick up, just call CIC/REQ or something and ask them to relay your request to the OT.
Actually a good idea.
It seems like you have a rudimentary understanding of the mortar operator. You are not obliged to simply sit and wait for coords like CAS. You can MAKE coords, unlike CAS (and more!). This post may be a little outdated, but explains it pretty well still.