Is it permissible to call a woman observing 'iddah merely to greet her?

There is no prohibition against speaking with a woman whose husband has died, whether the conversation is direct or by phone. There is no impediment to speaking with her, whether for important matters or merely to exchange greetings. However, if the speaker is a non-mahram, the conversation should be limited to what is necessary, and the speech should not be made soft.