My cellphone has a feature where you can send a text message to a home phone anywhere in Canada or the USA; the recipient’s phone rings and the system reads the text message aloud. The recipient can leave a voice message in reply. Link. 
Or both:
You can send e-mails to cell phones, they will receive it as a text message:
T-Mobile: phonenumber@tmomail.net
Virgin Mobile: phonenumber@vmobl.com
Cingular: phonenumber@cingularme.com
Sprint: phonenumber@messaging.sprintpcs.com
Verizon: phonenumber@vtext.com
Nextel: phonenumber@messaging.nextel.com
where phonenumber = your 10 digit phone number
And if you don’t want to give out your phone number to emailers, many systems will let you replace the phone number in your phone’s email address with a name.
Actually it should have said “where phonenumber=recipient’s 10-digit phone number.”
You already found a solution close to what I was going to propose, but what the hay…
If you send her a text message and she doesn’t have text messaging, you just cost her money and she will retaliate by calling and talking for two hours next time.
Someone already mentioned this but, my local phone company also has a number to call to access you’re voicemail. You could probably search out the number your phone company uses for this, call it, and use the menu to see if she uses the same company and if you can leave a voicemail with her phone number. Then she will find out you sent her a voicemail without calling her and retaliate by calling and talking for two hours next time.
My solution was to call her right before doing something important (or pretending to). “Hey this is StarvingButStrong. I’m at the hospital dropping off human organs for transplant and I wanted to touch base with you before I went into the ‘no cell phone zone.’ The caterer is XXX. Yep, they’re good. Yep, they’ll take care of it. I’m really sorry but I’m walking in the door now, gottagobye.”
Most systems have options but the operators and such are too lazy. For instance I worked one summer in college at a hotel and if the GM said “I’m out for the day, I’d put the call directly through to his voice mail.” It was fours steps. All the other operators said they couldn’t be bothered so they pressed his extention and hit transfer. This ment the GM’s phone rang three times then went to voice mail. My way it never rang and went into voice mail.
So I would bet there is a way but either people are too lazy to do it or don’t want to be bothered.
I know nothing about this service besides what is said in this article, but this service seems to do exactly what you ask for.There is a pretty good discussion in the comments.