There are plenty ways to PLL a RC oscillator with a Xtal reference even with low power. The timing can be done with a relativ low frequency low power device like in a GSM phone. It is also not a problem to make it failsafe, when the PLL fails for whatever reason it still transmitts with the RC frequency. The PLL loop can be made very slow and digital, which means that the XTAL oscillator is only needed for a small part of time. And if the xtal fails a relativ robust silicon RC oscillator can be used instead. With todays technology it is not very difficult, its more a question how much engineering you are willing to pay for.