Why does a small telephone receiver icon flash next to the WiFi symbol on my Lumia 550?

[Windows Central Question]

Why does a small old fashioned style telephone receiver icon appear next to the WiFi connection symbol, next to the signal bar in the top left hand corner of my Microsoft Lumia 550 phone? This little telephone icon replaces the arrow that points to the north west. The WiFi icon remains unaltered but the little receiver flashes on and off and then disappears
I cannot find an explanation for this. Is somebody trying to call me using the WiFi or WhatsApp or Viber?

 

[Maurizio Troso]

Why does a small old fashioned style telephone receiver icon appear next to the WiFi connection symbol, next to the signal bar in the top left hand corner of my Microsoft Lumia 550 phone? This little telephone icon replaces the arrow that points to the north west. The WiFi icon remains unaltered but the little receiver flashes on and off and then disappears
I cannot find an explanation for this. Is somebody trying to call me using the WiFi or WhatsApp or Viber?

It remembers me the call in progress or the missed call allert top icon

 

[xandros9]

I think TTY may be turned on. (under Settings > Ease of Access)

 

[Chintan Gohel]

Why does a small old fashioned style telephone receiver icon appear next to the WiFi connection symbol, next to the signal bar in the top left hand corner of my Microsoft Lumia 550 phone? This little telephone icon replaces the arrow that points to the north west. The WiFi icon remains unaltered but the little receiver flashes on and off and then disappears
I cannot find an explanation for this. Is somebody trying to call me using the WiFi or WhatsApp or Viber?

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