Spanish telecoms giant Telefonica is to increase its stake in Chinese counterpart China Unicom in a deal that will deepen the alliance between the two companies.Each company will buy $1bn worth of shares in the other. As a result, Telefonica's shareholding in China Unicom will rise to 8.06% while the Chinese state-owned mobile phone operator's will hold just under a 0.9% stake in Telefonica.Telefonica is paying HK$11.17 per China Unicom share while China Unicom is paying €17.24 for each share in the Spanish company.The two firms will cooperate on research and development and will also share infrastructure and equipment. According to the statement issued by the companies, the alliance will give both parties access to a combined global customer base of 550m.The announcement continues the trend of greater co-operation between global telecoms companies. Rumours persist in London that O2 mobile phone network owner Telefonica and Vodafone are teaming up to buy T-Mobile UK from Deutsche Telecom for around £3.5bn.Telefonica and Vodafone have an existing business relationship dating back to March when the two agreed to share antennas, base station sites and power supplies in Europe.