Analyst Gene Munster of ‘Piper Jaffray’ expects Verizon to be able to make a contribution of 2.5 million iPhone sales by becoming a partner with Apple. He projects that Apple can bring 2011’s US iPhone sales to 20 million, with Verizon’s sales largely coming at the expense of AT&T, AppleInsider reports.

In his talk, Munster uses this year’s numbers as a reference point. During the July to September period, AT&T sold 5.2 million units, which ‘Piper Jaffray’ estimates to be 80% of all AT&T’s smart-phone sales, and they go on to say that: “the iPhone has found continued success despite increased competition in the smartphone category and grew 65% year-over-year at AT&T”.

According to Munster, 2011 will result in 2.5 million more iPhones being sold with Verizon coming onto the scene – this means roughly 14% more than Apple could sell through AT&T alone.

An earlier report forecast AT&T selling 17.5 million iPhones next year, without any competition coming from Verizon, so this needs to adjusted as the new carrier will impinge on AT&T sales. Munster expects the new figures to be 11 million going to AT&T and 9 million to Verizon.

This year, AT&T is expected to sell 15.6 million iPhones, which represents 34% of global sales. Munster’s numbers reduce AT&T’s share for worldwide sales to only 17%, although global sales of the iDevice are expected to grow from this year’s 46.3 million to 63 million units next year.