I don't see a route to 172.16.98.x/24 in the configuration. Is there a gateway on 172.16.21.x/24 to that subnet?
Need to see the entire config. Sounds like there are some pieces are missing. If you are using it as an SBC then the devices on the 172.16.98.X network should not be talking to the public network at all they would be registering to the 908e and talking to it and it would then be acting as a B2UA and retransmit the voice to the carrier, which in that scenario the behavior you are seeing is expected. If you are using it as a router then you need to configure NAT so that it will be able to translate the non routable IP of the inside network to the routable IP on the outside network. In order to setup routing with NAT you will need to setup the IP Firewall create IP Policy Access rules, assign them to the interfaces, the one for the inside interface would have to have NAT source interface configured, but none of that is needed in the case of an SBC configuration.