To put a black phone in the Gmail signature instead of using unicode emojis use an image or a different character like ✆ (U+2706), dingbat telephone location sign.
Chrome Desktop for Windows doesn't support color emojis1, so it display black ones in some cases. By the other hand, and according to the OP, the new Gmail emoji support feature is changing unicode characters by its own version of hosted emojis. This also being done by others like Twitter and WordPress2.
- Read and test http://www.unicode.org/help/display_problems.html
- Try the
<p style="font-size:20px"> tags to prevent Gmail change the unicode character by something like
<img goomoji="1f4de" style="margin:0 0.2ex;vertical-align:middle;max-height:24px" alt="📞" src="https://mail.google.com/mail/e/1f4de">
In relation to my comment to the question, the first time that I read this question I was using Windows 8.1 and Chrome stable channel. The phone character in the first line was displayed small and black.
Now I'm using a Chromebook with Chrome beta channel, the phone character in the first link is displayed a bit bigger and yellow.
The above is, as was mentioned at the top, because Windows and other old OS doesn't support color emojis so maybe avoiding that Gmail change the unicode character by an emoji will not prevent that the unicode character change in other than Windows OS.
Copy and Paste Emojis 👍 Classic
This is the classic version of Get Emoji, showing backward-compatible emojis that work in all Windows 7
browsers, older versions of Android, and on Chrome for Windows 7, 8,
and 10. These emojis will show in black and white on older systems,
but will be converted to color when viewed by a recipient with a
system that supports color emoji. Switch to regular emoji to view all