Uhm, That will not just make it work.
1) You have to have your PBX WAN IP set in the "Outbound IPs" list on Flowroute
2) You need to route your DIDs to the WAN IP of your PBX (proper IP:Port)
3) Your trunk (be it Chan_SIP or PJSIP) has to be set as a PEER trunk. No registration, etc.
4) Your fromuser/fromdomain need to be correct.
5) You need to prepend your tech prefix to the outbound calls. Which is nnnnnn* not *nnnnnn (n being the numbers)
There is also a guide on their site for FreePBX, it's Chan_SIP only though. Knowing which SIP driver (Chan_SIP or PJSIP) you are going to use would make offering you steps to set this up a little easier.
But just adding the tech prefix to Dial() is not the only thing you need to do.