I do use BitRefill for one of my burner flip phones (not to mention all my food, Uber, and clothing stores). For that flip phone, which is Verizon, I pay about $35/mo via BitRefill, but again, compare that to Silent.link where you're paying only $2-$5/mo.
The problem with Silient.link is that you don't get a phone number. You have to use an XMPP service like JMP for that. Again, also very inexpensive, but still it's another layer.
In theory, the $35/mo burner phone shouldn't be necessary at all but unfortunately a lot of services won't send auth codes to a VoIP line (JMP) so it becomes necessary to have the burner on hand.
