47 CFR 10.510: Call preemption prohibition.

Devices marketed for public use under part 10 must present an Alert Message as soon as they receive it, but may not enable an Alert Message to preempt an active voice or data session. If a mobile device receives a WEA Alert Message during an active voice or data session, the user may be given the option to control how the Alert Message is presented on the mobile device with respect to the use of the common vibration cadence and audio attention signal.