Following AT&T's disclosure of its Q1 financial results, indicating more than 1.6 million iPhone activations, Piper Jaffray analyst Gene Munster has calculated that Apple may have reached sales of over 3.7 million iPhones overall for the quarter. Apple Store purchases and international shipments were combined with the AT&T number to estimate the total global sales.

“Last quarter, AT&T activated 1.9 m iPhones and Apple sold a total of 4.4 m units," Munster's note reads. “According to this relationship, the AT&T activation number implies a total of 3.7 m iPhones in the March quarter.”

If the iPhone sales estimate is accurate, Apple will have sold around 400,000 more devices than the Wall Street estimates of 3.3 million handsets for the quarter.

The iPhone appears to be one of the strongest products offered by AT&T, attracting subscribers that spend an average of roughly 1.6 times more than other customers. Approximately 40 percent of iPhone customers are new to the carrier, and less likely to jump ship after signing up.