If you are getting a voice prompt "Please enter your numeric message after the tone." The L or H will not work.
L is for low tone, H is for high tone. So without one of those tones, it will fail. It would probably be best if you just added several P's for pause, then a "C" for the outcall code as Ken suggested. Add enough P's to allow time for the pager to answer, then 2-3 more P's to make sure the pager prompts start playing before the "C" is sent.