I did some checking, and we ARE able to send phonemail messages to multiple 
locations.  When you are prompted to enter the extensions, use the following 
codes:

San Francisco: 415+XXXX (last four digits of the number)
Portland: 28+XXXXX (last five digits of the number)
Houston: 853+XXXX (last four digits of the number)

If you have problems, let me know.
kd