When you use a telephone, you are speaking to a person at the other end, not to the telephone. When praying at an image, icon, idol, murthi, you are not praying or speaking to that image, but to the deity it represents through the image. The image is a conduit. Sri Krishna says:
"The obstacles facing those devoted to the Impersonal Absolute are far greater; for the way of any unclear ideal is difficult for an embodied being to understand or follow." Bhagavad Gita 12.5
What this means is that with our limited (human) capabilities, it is difficult to connect with God who does not have a form native to this world. We need to use our senses, because we are sense-beings. Our minds are not developed to understand what we cannot see, touch, feel, hear.
The same verse with commentaries:
Bhagavad-Gita: Chapter 12, Verse 5
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