Our network is soemthing over 100 hosts and more than 700 services... keep in mind that much depends on the hardware you have and on the network performances when it comes to high numbers.
We have an SMS gateway box here.
Don't ask me HOW it works i don't know for me it's like a mail-server i send the email to firstname.lastname@example.org and get the SMS.
Many cell phone prviders have possibilities for commercial mail to SMS gateways ask them
Pay much attention while configuring.. it can get very expensive if you miss something and the net goes down and you have 2-3000 SMS messages in one night