Only Flext Premium accounts are eligible to accept outside numbers. Once you have upgraded to Flext Premium, click here to fill out the Flext port request form.
 
Once you submit this form, do NOT cancel your account with your current carrier or you will lose your number. It will be automatically canceled when your port is completed.

While the porting process is taking place, your current number still works with your old carrier.  Once it ports over to Flext, you will just receive your messages within Flext instead of your old system. So there is no reason to give out the temporary free number.  You can continue to give out your current number and it will always work. The destination of messages sent to this number is the only thing that changes.

Ports usually take between 3-6 weeks.