Isn't WhatsApp more popular? I understood it to be more popular and accessible while still using the Signal protocol.
More popular, but also closed source and owned by Facebook (make of that what you will)
who has publicly stated that they're mining the metadata, and does not by default notify you when one end's keys change (say if the phone were compromised).
Note that with cell phone spoofing, someone could impersonate you to OWS. All your contacts would get messages stating that your key ('secret numbers,' I think is the term they use) has changed, and all messages would then go to the imposter.
Do you have any info on how cell phone spoofing works? Is the IMEI number used by signal to verify you?