To fulfill this requirement, I created custom button and added action and enable rule to custom button.
This is working fine, when contact record has phone number in record and when that record opened button was shown.
But now requirement changed, customer wants to show button, as soon as Phone number entered.
If want to show button as soon as phone number entered, means I need to either save contact record or need to refresh ribbon.
I cannot save contact record because, there might be some other required attributes are missing.
With Xrm.Page.ui.refreshRibbon(); method, you can show / hide any button based on attribute value.