Have the sound man program his cell phone with an "old fashioned" ring tone.
As a sound man, NO. Just NO!
It's bad enough just following the queues for opening mics and adjusting gains through the production. This type of sound effect needs to come from the stage, not the sound board, and it needs to be handled by the stage manager.
Not to mention the time delay in setting up a call to a cell phone, it's never going to ring at exactly the proper time.