I assume that your PBX is running and calling from one extension to another works properly. If not, fix that first.
Flowroute can use either IP authentication or registration. IP authentication is generally more reliable and a little more secure. If the PBX has a dynamic IP address or an unusual network setup, registration may be required.
Assuming IP authentication and pjsip on the default port 5060:
Log into Flowroute and go to Interconnection -> IP Authentication. Note the values of SIP Proxy and Tech Prefix. If the public IP address of your PBX is not listed under My outbound allowed IPs, add it.
Create a pjsip trunk. On the General tab, fill in the Trunk Name e.g. Flowroute and put your main number (starting with 1 if US or Canada) in Outbound Caller ID.
On the Dialed Number Manipulation Rules tab, fill Outbound Dial Prefix with your Tech Prefix followed by *, e.g.
12345678* if your prefix is 12345678.
On the pjsip Settings tab -> General, set Authentication to None and Registration to None. Fill SIP Server with your SIP Proxy and SIP Server Port with 5060.
On the pjsip Settings tab -> Advanced, fill From Domain with your SIP Proxy. Click Submit.
Create an Outbound Route. Fill in Route Name as desired, fill Trunk Sequence for Matched Routes with your Flowroute trunk. On the Dial Patterns tab, fill match pattern with “XX.” (without the quotes). Click submit.
Click Apply Config.
From an extension, dial 1 800 437 7950. You should hear your caller ID read back. (With this simple setup, you must dial 1 before the area code and number.)
Once you get outbound working, adding inbound will be pretty easy.