You can use the Visible compatability checker to determine whether your specific phone is compatible:
https://www.visible.com/shop/compatibility
Most numbers can be transferred to Visible but see:
https://www.visible.com/switch-to-visible
What you might want to consider is signing up for Visible without porting your number in to first see if the service is appropriate for you. You can always port your number in at a later date. Some port on signing up only to find the service isn't right for them and then find it difficult to port back out. Yes you can keep you're contacts - those aren't tied to your service but rather your contacts app.
In fact, you can get a referral code from someone and get $20 off for the first month giving you the opportunity to try the basic service for $5 the first month. If you find it to be right for you do the port in otherwise just cancel (or let it lapse assuming you don't activate autopay).
Most active users here have a post (or two) in their profiles that provide their referral code info. Consider using one who actively participates in the community here beyond merely flooding the Savings Spot with their referral code whether me or someone else who's helpful - full disclosure we get a $20 referral credit when you use a referral code.
