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Many people withhold their numbers when phoning to ensure that calls don’t 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 “virtual phone number” to any subscriber who asks for one. Missed calls can be returned because the network redirects them to the user’s real number. But because the virtual numbers expire after a few hours or days, they cannot later be used for junk messaging.