Short answer

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.

Partial explanation.

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.

TODO:

1. Read and test http://www.unicode.org/help/display_problems.html
2. Try the <span>, <font size="10"> <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">

Remarks

Previous Comment

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.

Resourses

From http://classic.getemoji.com/

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 new emojis.

References

• ¹: Issue 333011: Windows 8.1 Color Emoji on Segoe UI Emoji is not supported on Chrome - Chromium
• ²: How We Set Up Emoji Support for Subject Lines - MailChimp Blog

When sending alerts for an entire Trello board to the specified Slack #channel, there is currently no way to limit said notifications to a single list within that Slack board.

The only "workaround" is to create an alert for a single Trello Card rather than the entire board. Assuming the Card has only 1 list the Slack channel will only get updates for that single list.

To do this, open the specific Card in Trello and with the Slack power-up enabled select "Send To Channel" or "Send via DM".

If your usecase is, for example, a Slack board where some users Care about the entire board's updates while others only care about updates made to a specific list or two, I would separate those users into 2 channels and use the above workaround.