Is there a metal that will not interfere with smartphone reception as a case?
That depends on what you mean by interfere. If you mean as if there were nothing there but air, then no. But if you mean the cell phone still works, then yes.

By putting metal near the phone, this metal will intercept some of the electrical field emitted by the antenna. Consequently, there will be some RF currents in the metal. These currents will circulate around, and ultimately:


be radiated again, or
go into losses, making something warm


It's pretty hard to prevent the energy from being re-radiated. As the currents wave through the metal, it's likely they will hit the edge. When they do, the current can't continue flowing, so a voltage develops. You have current, you have voltage -- sounds like an antenna.

In fact if you cut a slot in a waveguide with the right dimensions, it makes a great antenna. It's called, unsurprisingly, a slot antenna.

Depending on the geometry and placement of your metal, it might re-radiate the energy it intercepts quite efficiently. Or, it might not radiate so efficiently, and instead a lot of the current will circulate in the metal and be absorbed by resistive losses. It's going to depend a lot on the specific geometry of the metal.