*** This is not a recommendation regarding a specific router brand, and I have no experience with Google Mesh ***
Voice calls (SIP) perform an RTP port 'negotiation' through the SDP header. The double-NAT is not a problem as long as the first NAT device (Google Mesh in your case) is not SIP-aware (meaning that it sees the SIP packet and adjusts the SDP header). If you have this possibility, disable any SIP related features on the local router.
On the FortiGate, make sure the default settings are used (SIP-ALG is enabled).
- Toss a 'Like' to your fixxer, oh Valley of Plenty! and chose the solution, too00oo -