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This is a slippery slope issue. However, if the cell phone carrier attempts to collect federal taxes as part of its lawsuit, there is a possibility that the suit then gets tossed. If the carrier sues ONLY to collect its charges and no federal taxes or tariffs, then you have to rely on the state statute of limitations.
There are two competing opinions regarding this issue. One says that it does not apply out of the 5th Circuit Court of Appeals. The other opinion argues that it does, but is not very analytical and that one comes out of the state of Illinois.
Scrutinize the charges that are asserted as part of any lawsuit or claim. Then proceed from there. If there are any tariffs or federal taxes included in the bill, then its probably limited to 2 years.
Do not automatically assume this is the only statute of limitations. Apply your state's SOL too.