[How-to] Install Freepbx distro (with commercial modules) on Google (cloud) Compute Engine

What driver are these extensions using? Chan_SIP or Chan_PJSIP? If Chan_SIP then under the Advanced tab you need to set the NAT option to Yes. If Chan_PSJIP, it’s NAT aware and will deal with it.

What type of router is in front of these phones?