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Many people withhold their numbers when phoning to ensure that calls don鈥檛 lead to a torrent of cold calls and/or spam text messages. But that also means recipients who miss a call cannot return it. So in patent application W0 2004/34713, the Finnish firm Nokia suggests phone networks issue a temporary, limited-life 鈥渧irtual phone number鈥� to any subscriber who asks for one. Missed calls can be returned because the network redirects them to the user鈥檚 real number. But because the virtual numbers expire after a few hours or days, they cannot later be used for junk messaging.