How to specify extension number in `phone` and `contact:phone`?

During my mapping journey, I have encountered several places which are perfectly notable as POI, but I have problem recording their phone contact information on OpenStreetMap as they made use of extension number; Wiki entry on phone and contact:phone key does not mention extension number at all.

For example: Art Library, which is located inside Bangkok Art and Culture Centre, has following phone number:

+66 2 214 6630 ext. 126

Dialing +66 2 214 6630 alone will lead you straight to Bangkok Art and Culture Centre's main PBX. While one can ask the operator to manually transfer the call, it is time-consuming and defeats the purpose of documening phone number on the library POI to begin with (it is equivalent to calling Bangkok Art and Culture Centre- the enclosing feature). On few other places I have visited and called, there are no human operator present at all; only voice message This is (...), please dial an internal extension number is played.

According to (1) and (2), default dialer program in Android and iOS seems to support special Pause numbers in contact entries, which are used to separate main number from extension number:

• , (comma) can be added to introduce ~3 sec pause before DTMF-dialing the rest of number.
• ; (semicolon) can be added to show confirmation prompt (time-unspecifed pause) before DTMF-dialing the rest of number.

As semicolon is used as OpenStreetMap's value separator, p and w can be used instead of , and ; at least on Android dialer (not sure about iOS though), unless one is willing to use a less-readable escape sequence.

So, I would like to know...

• Are there any proposal, concensus, or relevant standard document to look at, for ways to specify phone extension number on OpenStreetMap data attribute?
• Does mobile-based OpenStreetMap programs (e.g. OsmAnd and Maps.me) react correctly to phone number written with these kinds of separator?
• Does dialer on both Android and iOS dial them properly (with pause/prompt) when invoked from map programs?
• Does p and w separator work correctly on iOS dialer when invoked from map programs?

Side notes:

• RFC 3966 a.k.a. tel: protocol apparently allows specifying extension via ;ext=, so the above number could be written as this tel: link: tel:+6622146630;ext=126 (It still clashes with OpenStreetMap's semicolon separator, though)
• Some feature phones allow p and w separators in contact entries as well (though not , and ;).