The safest way is to buy sim cards. I have about 7 of them that i use + a few google voice numbers + a few family members, and its the best way.
But for example the cards have an expiration date if credit is not added. So every couple of months i add $2 (minimum allowed) per card in order to renew the expiration date. I feel like a drug dealer on the kiosk adding credit on 7 sim cards xD
And of course the google voice numbers expire dur to inactivity also. But i get an email i believe 30 days before expiration for eacch number, so i just go in and text the numbers between them, so it refreshes the expiration date.
Its good to have a few sim cards for online work. You know you wont lose the numbers. You know no one else is using them for sure. Its a good thing.