Please define growing rapidly. 30 sites in 2002, 40 in 2003, 50 in 2004, 70 in 2005. 300 phones in 2002, 400 in 2003, 500 in 2004, 700 in 2005.
If you have projects scheduled for 2006 for implementation of new sites and you need all current personel for those implementations, block out those dates for other service on the calendar. If projects will tie up all your people for (9) 2 week periods during 2006 then make that known. Request those periods to be scheduled around for all other requests. If your numbers are realistic, and the reduced availability for other things is not acceptable, then you have established a problem which needs to be addressed. New staff may not be the solution, temp, contract, or third party providors during your peak times may be a better fit.
Do not tie your hands, or your argument into needing new employees, establish the projected work load, and projected response times, and projected projects causing windows of black out for other services during implementations, or projects. If these parameters are acceptable, then you may not need additional help.
Finaly, if you are given unrealistic work loads and deadlines, say so, in no uncertain terms. If you allow them to stand unchallenged, cut corners, or give up having a life in order to meet them, you have yourself to blame. If you make them known as such, then you are not responsible for the outcome if things do not meet deadlines. Sending an email to a project manager, documenting that timelines can not be met with current staff and keeping the email is a wise course. Also, make a 110% effort to show your commitment was not an issue.
You do not always get what you pay for, but you never get what you do not pay for.