I have a few cell phones that used be with Tmobile, ATT, and Alltel. I've seen ads that say buy a Motorola sim card for unlimited everything $50. If I put that simcard in my Motorola Razor would it work again?

If the handset isn't locked to those three networks i.e. Tmobile, ATT and Alltel, then any sim card will work. Almost all handsets are locked to certain networks and so we can't use sim cards of other networks. You can verify this in only one way. If you have a sim card of another network (in your hand at the moment or you can borrow it from your friend or from other family members), you can insert the sim card, suppose a Verizon sim card, not those three. If the verizon works, then all sim cards should work on your handset.

Since your phone already works with Tmovile, ATT and Alltel, I hope all other network sim cards should work without any problem. Suppose if a phone is locked to network means, it's locked by the Service Operator to preserve their customers so that the customers will return to them if anything happens with the sim card or with the phone, this is a business plan logically designed.

For now, I can't confirm you, but at least if you could try with another sim card of another network (verizon) and it works, the no doubt, all sim cards should definitely work. But if you can't try it that way, I wouldn't recommend you, until you get it tested.
And that Advertisements seems a kind of something, "....unlimited everything...." sounds good, but must be unlimited Incoming Call and could also be without monthly rental fees. I don't recommend unless you ask about it to someone who already used the same sim card ($50). As we know, advertisement is advertisement.

If you need further help, let me know.

Good luck.

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