Line stays off-hook

We are using cell phone VOIP phones as our extensions. Our problem is that sometimes, due to the phone going out of service, or maybe switching from one access point to another, the phone gets disconnected and then the phone is off hook with no extension attached. The only way to solve this is to reboot the freepbx computer, since an extension is not able to connect again due to the line being busy.
1.- Is there a way to get the phone line on hook without rebooting?
2.- Is there a setting that makes the phone stay operational so that after the momentary disconnect it is operational again?