I'm not sure where you are located, but when I researched them late last fall for a USA install, I had a hard time finding a SIP door device that was available in the USA market. I considered analog, but then in the end we installed a wireless doorbell instead.
This request has come up before. Do a search through the forum, I seem to recall someone providing some links to a system that they had used/recommended. If you are talking about a multi-tenant (apartment building type system), then it might be less expensive to go with a conventional analogue unit and an ATA.
The 2n phones are the nicest looking ones I have seen. Valcom makes good products, we use their paging equipment but have never used one of their door phones.
I don't know how true ... but the UK Sales guy for CyberData has told me that their SIP door entry unit is 3CX compatible. Looks to be a good strong unit with or without an entry control keypad ...