I'm personally a fan of mushroom hooks (3M Dual Lock) besides the high price, because it's less spiky than Velcro and has more lateral rigidity (though unfortunately you need 3 pieces on each side in a triangle for a robust stable mount free of tilting wobbling). Though I actually wouldn't recommend it for mounting a phone; I added a piece to my phone case, but took it off because it kept catching onto my hand and surfaces, and ended up with more entangled dust than soft Velcro.
I actually stuck the velcro to a cheapy Otterbox rubber sleeve knockoff rather than the phone, which has some weird sticky-thing-repelling coating that the Velcro wouldn't stick to.
