Wow, did not think it would be that basic. Thank you, that did the trick.
Add support for SIP addresses
Store SIP addresses of data kind CommonDataKinds.SipAddress
(called “sip_address” in the edit schema) as sip/sips URI’s in TEL properties.
“sip” is added to URI’s without scheme. Android allows a maximum of one SIP address for each vcard.
Requires update of ical4j-vcard to allow Telepone properties containing a URI: ical4j-vcard-telephone-uri-mikma.diff
It seems there are several incompatible conventions for SIP URIs in vCard: X-SIP is also used. Which clients put SIP URIs in the TEL field?
Actually I didn’t have a particular client in mind when implementing this feature, but wanted it to be vCard 4.0
compliant. I think RFC 6350 is pretty clear that for URI’s which are used for voice communication TEL properties should be preferred, and the SipAddress data kind in Android is only used for voice calls AFAIK. (The SipAddress data kind is displayed as “Internet Call” in the standard contacts app “People”.)