(Reuters) - Japan's SoftBank Corp will soon downsize its Silicon Valley offices, people with knowledge of the matter said, signaling the company won't revive efforts to buy T-Mobile U.S. Inc.

SoftBank subsidiary Sprint Corp dropped its bid to acquire the No. 4 U.S. carrier in August but the companies did not rule out future consolidation.

The Japanese telecommunications companyhttp://images.intellitxt.com/ast/adTypes/icon1.png is now transferring "the bulk" of manpower out of its West Coast operations, including dispersing development engineers to Sprint headquarters in Kansas, said the people, who declined to be identified because the move has not been made public.

SoftBank is also considering renting out one of two buildings it leased at an annual cost of over $3 million to accommodate a T-Mobile-driven expansion, the people said. The building has stood largely empty, they said.

The failed bid by Japan's acquisitive No. 3 mobile carrier was a rare setback for founder Masayoshi Son. The billionaire encountered resistance from U.S. regulators, who insisted on keeping the number of major wireless carriers at four.

"There were people sent to Silicon Valley for the purpose of making (mobile phone) platforms, but that job was done and there's nothing else to do," said one of the people.

SoftBank spokesman Matthew Nicholson said some SoftBank employees are moving back to Tokyo or going to Kansas as certain joint projects between the company and Sprint have finished. He declined to comment regarding the relationship between the departures and the failed bid to acquire T-Mobile.

SoftBank bought No.3 U.S. carrier Sprint last year for $22 billion as part of an overseas expansion that has included investments across Asia.

Son had seen the acquisition of T-Mobile as key to taking on U.S. market leadershttp://images.intellitxt.com/ast/adTypes/icon1.png AT&T Inc and Verizon Communications Inc.