Choosing between Host Based Routing and SIP Registration
MiRTA PBX doesn't support natively SIP registration for Providers, but you can manually configure them in sip.conf
Here a simple example about a static registration in sip.conf
[general] ... register => 49088751:cyj74rrjd8gg@sip.flowroute.com
If you want, you can also define statically the SIP provider in sip.conf and then reference them in the Admin/Providers section. Doing a static configuration should be avoided because any change in the provider configuration will require a reload, but it can be needed if a parameter not yet supported by the web interface needs to be used.
[flowroute] type=friend secret=cyj74rrjd8gg defaultuser=49088751 username=49088751 fromuser=49088751 host=sip.flowroute.com dtmfmode=rfc2833 context=fromoutside canreinvite=yes allow=ulaw allow=alaw allow=gsm insecure=port,invite fromdomain=sip.flowroute.com
In the web interface, you'll reference the tag in square brackets [flowroute]
Once sip.conf has been edited, to have it working, doing a sip reload is needed. Take in mind doing a sip reload will trash all clients registrations. Phones will be unavailable until new registration.
Before choosing to go with the SIP registration, please take the time to read
https://blog.flowroute.com/2014/07/09/sip-registration-vs-host-based-routing-which-side-wins/