iPhone 4 - Unable to Access Incoming Call While on an Existing Call

[Nelais]

I have been experiencing for the last 3-4 days not being able to retrieve or clicking over to a waiting call while on an existing call. The icon to switch calls, nor the indicator "Multiple Calls" does not appear on my screen when another call comes in. Has anyone else experienced this, and if so, how did you resolve this issue? Thx.