Yes, we can configure our virtual SIP PBX to fit this requirement. Of course, please sign into your account firstly.
Please click menu "Data / system / Basic". In the configuration page, please change "outgroup call prefix" to '8', then click button "update" to save your modification.
outgroup call prefix = 8
Then, when we dial "8xxxx", calls will be sent to outside through external line and outgroup-prefix '8' will be deleted finally before the called parties "xxxx" receive calls.