Oppenheimer & Co. Telecommunications Analyst Tim Horan believes Apple will make the iPhone available to all major carriers in the next 18 months. This would effectively double or triple the company’s iPhone sales.

According to his research note, T-Mobile will get the iPhone this summer, followed by Verizon and Sprint in the fall and Clearwire in 2011.

“We believe AT&T’s iPhone exclusivity arrangement with Apple will be expiring by mid-2010,” Horan wrote. “Fore wireless carriers, customers are demanding the device and they need to remain competitive.”

With this strategy of signing on every carrier in the US, Apple will increase its sales by 15 million to 25 million units sold annually. Obviously carriers would have to respond back with a robust network, and between the aforementioned companies, it could take $3 billion worth of upgrades to support the increase in data usage.