Well, that depends.
Do you want to keep the same telephone number? If so, you will need to contact Phonepower to port the number over from Vestalink. If you don't care about the phone number, then just get a new number from Phonepower.
Log into your OBiTALK dashboard, here:
http://www.obitalk.com/obinet/pg/obhdevClick on the listing for your OBi 110. Then, look at SP1 or SP2 -- whichever one is now configured for Vestalink. Click the trash can icon to delete the configuration. After it is deleted, click on the SP1 or SP2 row again, to configure it with the credentials you will get from Phonepower.