How to process a multiple values field (comma separated) in a Rules Action?
The rules action calls sms_send, whose first argument is:


  The destination number.


i.e. singular. So the module doesn't have support for sending to multiple, comma-separated numbers in one go through rules. It is in alpha to be fair, so it's not likely to be fully-featured yet.

Two ways you could get around this limitation (there are probably more):


Create a custom rules action which splits the incoming string by comma, then loops over and calls sms_send on each. If you choose this method, contributing the code back to the project would I'm sure be greatly appreciated.
Restructure your workflow so the existing rule is invoked once, in a loop, for each phone number.