Monitoring and recording is interrupted in Apple GarageBand and other Audio Apps from the ONE for iPad & Mac, Duet for iPad & Mac, Quartet, JAM, JAM96k, MiC, or MiC96k when a call is received. Once the call has ended or declined monitoring and recording is resumed.
Unless calls are more important than creativity? It is recommended that in the iPhone> Settings Do Not Disturb is turned On. This will prevent a call from interrupting that perfect take or inspired performance.