A system comprising: a companion device; and an accessory device, the accessory device comprising: at least one antenna for performing wireless communication; a first radio coupled to the at least one antenna, wherein the first radio is configured to perform cellular communication with a base station; a second radio coupled to the at least one antenna, wherein the second radio is configured to perform short-range communication with the companion device; and one or more processors coupled to the first and second radios, wherein the one or more processors and the first and second radios are configured to perform wireless communications using the at least one antenna; wherein the accessory device is configured to: operate in a first mode where the first radio of the accessory device is configured to perform cellular communication directly with a base station; operate in a second mode where the second radio of the accessory device is configured to perform short-range communication with the companion device, and wherein the accessory device is configured to utilize cellular functionality of the companion device to provide cellular communications through the companion device to the base station; characterized in that the accessory device is further configured to: receive signaling provided to the accessory device by the companion device regarding battery power and current cellular communication capabilities of the companion device and, in response to the received signaling, selectively and automatically transition between the first mode and the second mode based on two or more factors comprising the battery power of the companion device and whether the companion device currently supports cellular communications. a companion device; and an accessory device, the accessory device comprising: at least one antenna for performing wireless communication; a first radio coupled to the at least one antenna, wherein the first radio is configured to perform cellular communication with a base station; a second radio coupled to the at least one antenna, wherein the second radio is configured to perform short-range communication with the companion device; and one or more processors coupled to the first and second radios, wherein the one or more processors and the first and second radios are configured to perform wireless communications using the at least one antenna; wherein the accessory device is configured to: operate in a first mode where the first radio of the accessory device is configured to perform cellular communication directly with a base station; operate in a second mode where the second radio of the accessory device is configured to perform short-range communication with the companion device, and wherein the accessory device is configured to utilize cellular functionality of the companion device to provide cellular communications through the companion device to the base station; characterized in that the accessory device is further configured to: receive signaling provided to the accessory device by the companion device regarding battery power and current cellular communication capabilities of the companion device and, in response to the received signaling, selectively and automatically transition between the first mode and the second mode based on two or more factors comprising the battery power of the companion device and whether the companion device currently supports cellular communications. at least one antenna for performing wireless communication; a first radio coupled to the at least one antenna, wherein the first radio is configured to perform cellular communication with a base station; a second radio coupled to the at least one antenna, wherein the second radio is configured to perform short-range communication with the companion device; and one or more processors coupled to the first and second radios, wherein the one or more processors and the first and second radios are configured to perform wireless communications using the at least one antenna; wherein the accessory device is configured to: operate in a first mode where the first radio of the accessory device is configured to perform cellular communication directly with a base station; operate in a second mode where the second radio of the accessory device is configured to perform short-range communication with the companion device, and wherein the accessory device is configured to utilize cellular functionality of the companion device to provide cellular communications through the companion device to the base station; characterized in that the accessory device is further configured to: operate in a first mode where the first radio of the accessory device is configured to perform cellular communication directly with a base station; operate in a second mode where the second radio of the accessory device is configured to perform short-range communication with the companion device, and wherein the accessory device is configured to utilize cellular functionality of the companion device to provide cellular communications through the companion device to the base station; characterized in that the accessory device is further configured to: receive signaling provided to the accessory device by the companion device regarding battery power and current cellular communication capabilities of the companion device and, in response to the received signaling, selectively and automatically transition between the first mode and the second mode based on two or more factors comprising the battery power of the companion device and whether the companion device currently supports cellular communications.