FWIW, this is what flowroute writes about Tech Prefixes:
Tech Prefix
What's This?
The tech prefix is used if you choose to use IP-based authentication for outbound calls. To use it: define your outbound allowed IPs below, then prepend the tech prefix before each call.
Format for NANPA(USA/Canada) calls must be 11-digit.
Format for International calls must be Country code followed by number, no 011 or 00 required.
Example: To call NANPA (USA) - +1 310 555 1234, send the call to "MYTECHPREFIX#13105551234@sip.flowroute.com".
Example: To call International (France) - +33 1 5871 1234, send the call to "MYTECHPREFIX#33158711234@sip.flowroute.com".
SIP Proxy
sip.flowroute.com
Prefix
MYTECHPREFIX#
IP-based authentication is recommended for users with static IP addresses. However, if you are behind firewall or NAT router, you must use SIP Registration.
If you need assistance configuring your PBX or VoIP system, please visit our System Configurator.