If you are sure that the phone included, call him from a landline or another mobile phone. The sound is easy to identify where it is located.
If the phone is turned off, unavailable, or set to silent mode and the alarm signal, start the search manually. Check first the places where usually put the phone: shelves, drawers, bags, backpacks. Remove everything, check the lining and the space around, then put the items back.
Check all the pockets, especially things that have washed. Do not forget about the lining.
Go to the places where I leave the phone rarely. Similarly, remove everything from the drawer or off the shelf, check in and around, and then put one item back. Then exactly the same make where the phone could not be.