Verizon Wireless offers free parental filtering on all their phones, EVEN THEIR PDA PHONES for adults which offer web access. you just have to call up using 611 on your phone, or 800-922-0204 from a landline phone. Press 3 for tech support, say “filtering”, and wait for the rep. Tell him you want content filtering (set to “teen”- “child” may block some important things), and they’ll do it for you free.
I do not know if other wireless companies offer this service. If they don’t, and you need a data plan, it’s worth it to switch to Verizon wireless for the extra shemiras einayim.