I know you said your battery is fully charged, but the beeping is an indicator that the battery is low and soon will cut off.
Is your unit old? If so, it's possible that the batteries are not holding a charge any longer.
Is that happening on all three phones? If so, are they all old phones with original batteries?
What does the power indicator show? It's a small icon that looks like a battery on the screen. If it's down to one bar, your battery is low.
If any of this is true, you need to replace the batteries with the same rechargeable type.