How do I include extensions in the tel: URI?

I currently have a webpage serving up phone numbers, some of these phone numbers have extensions so I have written the HTML like this:

<a href="tel:+44-1234-567;ext=88">+44-1234-56788</a> / <a href="tel:+44-1234-567;ext=99">+44-1234-56799</a

When I hit this page in my Android browser and tap one of the phone numbers, it loads up in my phone dialer (UK Samsung Galaxy s2 stock) as:

+44-1234-567;ext=88

which I don't think is correct. Surely it should omit the ;ext= word.

Have I misread the RFC for implementing tel?

Seems the proper way to do it is use a comma:

<a href="tel:441234567,88">+44-1234-567 ext.88</a>

Just tested with iPhone and Android OS 2.1. Using ;ext=88 converts the ext bit into a number that is dialed with the extension (so it dials something like 35888 instead of 88).

According to the documentation, you can add what you want like so 12345678;ext=123

See RFC 3966

Comment for How do I include extensions in the tel: URI?

As of June 2021 the RFC3966 ;ext= syntax still isn't implemented by Android, and it's inelegantly implemented by iOS.

Using ;ext=123 as an example:

• Android: after the call connects a modal window appears asking Send the following tones? 396123 with No and Yes buttons. "Send the following tones?" is a precise technical description of what will happen if the user taps Yes, but it is probably not the best wording for the average user.
• Android converts ;ext=123 into 396123 because it treats the letters the same way as if you were dialing something like 1-800-FLOWERS, and this is a broken implementation of the syntax.
• iOS provides an option to the left of the Disconnect button that says Dial “ext=…”. When you tap on this "button" it will dial the extension number. This is inelegant and has bad usability because the "button" doesn't look like a button — it's just plain text — and because you can't see the extension number.
• In addition, when you first tap on a phone link in iOS it presents a button at the bottom of the screen which partially rewrites the phone number into a local format, but which also preserves most of the ;ext= syntax, e.g. Call +1 (555) 555-5555;ext123. This is also inelegant, and it's ugly besides.

If you instead use just a ; which is supposed to mean "wait," as in "wait until the auto attendant message ends and then automatically dial the extension":

• iOS: tapping the link displays a button stating Call +1 (555) 555-5555;123 which is slightly less ugly than the button described above.
• iOs provides the same extension-dialing "button" described above except the extension number is visible, e.g. Dial “123”. It still has the other usability problems.
• iOS does not automatically dial the extension after the message ends.
• Android: after the call connects a modal window appears asking Send the following tones? 123 with No and Yes buttons.
• Android does not automatically dial the extension after the message ends.

So for now, as of June 2021 it seems that the only way to include extensions in tel: links that will actually work is to use either ; for "wait" or , for "pause":

• <a href="tel:+1-555-555-5555;123">555-555-5555 ext. 123</a> — this will provide a UI component which the user can invoke to dial the extension. The usability of the UI component depends on the OS; neither are great, but Android's is arguably better.
• <a href="tel:+1-555-555-5555,123">555-555-5555 ext. 123</a> — this will automatically dial the extension a couple seconds after the call connects. Note: This mechanism will not work with voicemail systems that don't accept user input until the auto attendant message ends.

In all the examples I saw, the value of ext is contained in the full number. So try including 88 in the href value:

<a href="tel:+44-1234-56788;ext=88">+44-1234-56788</a>