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Known impOS Issues With Twilio Super SIM

impOS™ supports the use of Twilio’s Super SIM with the imp006 module. This requires impOS 42 or above. However, because of the use-cases that originally informed the design of impOS’ cellular functionality, there are a couple of issues of which customers working with imp006 and Super SIM should be aware when designing their products.

  1. impOS may power-cycle the cellular modem before Timer-triggered Super SIM IMSI switches take place
    Super SIM uses a timer to trigger IMSI switches. The timer period depends on the version of the Super SIM IMSI switcher applet. Values range between three and 20 minutes. By default, impOS power cycles the modem after five minutes of continuous registration failure, though an application may specify a shorter timeout for the entire connection attempt. This means that, for some Super SIM applet versions, the IMSI switch timer will likely not fire and IMSI switching will only occur in response to other scenarios.
  2. The IMSI presented by the imp API is only sampled at modem power-up
    impOS’s cellular support was originally designed for devices containing a single-IMSI MFF2 SIM. This means it does not specially handle either the hot-swapping of SIM cards, or SIMs of any form-factor that do IMSI switching, ie. Super SIM. As a result, the IMSI exposed in the data returned by is not updated when either the SIM is hot-swapped, or Super SIM switches IMSI. We therefore recommend that customers using Super SIM do not make use of the reported IMSI. The ICCID, which is available from the same API, is a better choice for identifying a specific SIM — and as a proxy for the device ID if you are using MFF2 Super SIMs.