Why only US mobile numbers?

[Emily]

You have the option of selecting different time zones, so why must you
have a valid US phone number to use this application? This invalidates
this very useful application for everyone whose mobile is not US
based. This application would be especially useful in the immediate
aftermath of an earthquake, such as the Haiti and Chile earthquakes,
as it was designed to be used, though many of the people would not be
able to sign up for this free application. This needs to be fixed to
reach a broader range of users.

Thanks,
Emily

[Ted Morse]

Hi Emily:

Currently the US-based restriction is only a limitation of the
software checking whether a phone number is well-formatted. However,
should we allow any number, we'd incur charges by the user's mobile
carrier for sending/receiving international SMS -- currently we only
have a US-based number for users to send messages to. In the future
it would certainly be nice to have international numbers, but we
haven't reached that point, yet.

You are certainly correct that removing this limitation would be a
huge boon for potential uses all over the world. Furthermore, adding
the ability to sign up by SMS alone would enable an even broader range
of users in places where internet access may be limited.

Thanks,
-Ted