Asterisk just released 16.29.0, 18.15.0 and 19.7.0 that now have Tel URI support for Chan_PJSIP. It will now accept Tel URI incoming requests, however, Asterisk still cannot generate Tel URI headers for outgoing requests.
Also, 16.29.0 is the last supported released of v16 which is now officially Security Fixes Only.
There will be one more release of both 16 and 19 which will include big fixes. This is because we cut these as release candidates over a month ago, so there was still time for bug fixes to accrue before things went security fix only.
The main reason I posted this was because there are still those who have yet migrated because their providers send incoming calls to them using Tel URI. In that context/need, I don’t think REFER requests are a concern.