Piranha has changed rates
 

Piranha seems to have lowered their USA rates a bit.
The T-Mobile ones are lower.
Frankly, I do not recall what the AT&T rates were.
Also, they are the only two networks showing for the USA.

They are the only 2 nationwide GSM carriers. There are a couple that serve just local areas.

At one time they showed Telna and an Alaskan GSM provider.
The Alaska rates were more. I believe that they use an agreement with Telna to access T-Mobiles network.
The UK number they provide shows up as Telna also.

The UK number results:
Number billable as mobile number
Country or destination United Kingdom
City or exchange location
Original network provider* Telecom North America Mobile Inc

I see Piranha have reduced the Voice/Data rates in EU + countries, and are going to reduce in Asia/South America shortly, i just checked the rates out and some prices are very good. Cheapest PAYG rates i have seen so far. Do you think others will follow suit.