A cell phone under the bed is a sign that whoever cleaned the room didn't look under the bed. The maid should have found that phone. Possibly describe it as "an item left by the prior guest" and don't say what it was?
The filthiest hotel I ever had the misfortune to stay in was a Motel 6 in Washington DC. The beds were horrible and lumpy and in the morning when looking for a shoe, we found a toilet paper core and a whisky bottle under one of the beds. There was something else too, but I no longer remember what it was. I'm thinking something like dirty kleenexes maybe.
We got refunded for that night's stay and got a voucher from corporate for another free night after we made appropriate complaints. Be glad it was just a cell phone.
We moved to the Super 8 next door that night. $3 more per night and the rooms were spotless.
(We were on a cross country trip and had booked at Motel 6's all across the country for the entire three week trip. That was the only filthy one we encountered. Normally they are clean enough.)
Time to build a bigger bridge.