This issue is common with Voip and porting. Voipo is routing your calls to your sister through their network only because her number is now in their system. IMO, Russell has the best answer--have your sister set up her PAP2 and forward CallVantage calls to the Voipo temporary number assigned. (If she unplugs her CallVantage TA, she can use the Safe Forward Number option.)

The only potential issue is that when the port actually occurs, your sister may not receive calls properly until her Voipo ATA is re-provisioned (and possibly rebooted). Setting a failover number (cell phone) in her vPanel will address that concern.