What are the rules for putting a full stop after an abbreviation.
For instance, I want to say the following on my business card.
Do I need to put a full stop after the Tel?
On a business card, Tel: should be abundantly clear without the need for a [BrE. full stop | AmE. period]. The overriding concern is clarity. If an abbreviation could be mistaken for a word, include it; otherwise, you can probably remove it.
The trend has been to simplify. As a matter of rule, measurement units are bare (except inches which is always in. so as not to be confused with the word in). So too are Canadian and U.S. postal abbreviations. Most style books and corporate identity guides prefer the bare form for acronyms as well: it is NATO or Nato and no longer N.A.T.O., IBM no longer I.B.M. And in Britain, Saint is commonly St now, although universally St. in the U.S.
On the principle of the less punctuation the better, no. Alternatively, you could put 'Telephone' in full, or insert an appropriate image.
The full-stop is necessary to make it clear that it is an abbreviation for 'telephone'.
In your example, as you already have a colon, the full-stop may be dispensed with, without ambiguity. In all other cases, it is necessary.
The rules differ between AmE and BrE.
In BrE a full stop is needed unless the last letter of the contraction is the same as the last letter of the original word, e.g. Rd (Road), Mr (Mister) and St (Saint or Street) do not need one, but Prof. (Professor), Rev. (Reverend) and Dept. (Department) do.
In AmE the rule is to always use a full stop/period, regardless of the last letter.
However, there is a growing trend to omit punctuation - except in sentences, of course! You may notice the absence of commas after each line in (street) addresses, for instance. This is called "open punctuation" and has existed for at least 25 years. Using the punctuation in abbreviations and so forth is known as "full punctuation".
Personally (a Brit), I would omit the full stop and the colon as well.