I think pagica and numericpage would work. Basically a gateway that forwards numeric pages to your BlackBerry. And Cingular's Enterprise as well. It seems like you already covered all the bases -- I guess you're shopping around for other options?
If you are trying to save money by avoiding pagica or other third party, use the 7520 model - because if you are on Nextel, did you know Nextel already provides free numeric paging with your BlackBerry. (For instructions, refer to Nextel, or search for 'nextel paging')
Some carriers provide a special phone number that allows you to send a numeric page to a BlackBerry (all models), and Nextel is one of them.