### Description - Type: Bug - Priority: Blocker With the introduction of #4389, `UARTSerial()` hits the `Mutex not allowed in ISR context` trap. **Expected behavior** Transmit serial bytes. **Actual behavior** The call stack is as below, basically `UARTSerial()`’s `tx_irq()` is calling `SerialBase:writeable()` which has a mutex in it.  **Steps to reproduce** Use the new `CellularInterface`, which uses `UARTSerial()` to talk to the cellular modem.