Google has allowed paying for Play Store purchases through your phone carrier's bill for a while now, but it's not available on all carriers. Vodafone in Portugal just started supporting Play Store carrier billing, becoming the first carrier in the country to do so. In fact, the Google Play support page for Portugal hasn't even been updated yet.
If you've never used Play Store carrier billing, it's extremely simple. When purchasing an item, simply select your carrier as the payment method and press Confirm. Carrier billing in the US is supported with AT&T, Sprint, T-Mobile, US Cellular, and Verizon.
If you're a Vodafone customer in Portugal, you should see the new payment option right now. Carrier billing on Vodafone draws from your account balance first, then bills you for the remainder.
Source: Vodafone Portugal
Thanks: David Monteiro, Diogo Dinis