"SIM card rejected"

[wco81]

I've kept the Poste Mobile SIM in my phone here in the US. It uses the AT&T network, which is the bigger of the two GSM networks in the country.

Today I got a text:

se non ricarichi o non utilizzi i Semplifica a pagamento entro i prossimi 30 gg la tua SIM sara disattivata (art 4 cond. contr.). Info 803160

Google translation wasn't the best but I gather it's saying recharge or the SIM will be de-activated within 30 days?