After what I've seen of O2's IMS, I really do not trust it. Not one bit.
I don't know exactly how reliable it is for ported numbers as I only know 1 person with a ported O2 number. He seems to fair OK and haven't really heard any complaints, but the O2 IMS service it's self is horrific.
I know a couple people now who have just been booted off VoLTE for no good reason, WiFi calling bugs, dropped calls, international dialling for no reason when in the UK; just to name a few.
These are issues I haven't seen with any other IMS. EEs has robot voice but that can be fixed mostly by ending the call and dialling back out again.
Three and Vodafone IMS seem the most stable in my opinion, Vodafones being the best call codec wise (obviously the EE to Three incoming call bug exists unfortunately).
I also highly rate Vodafone for calls because of their HD Voice over 2G.
That's why, if someone asks me what network to use if their No1 priority is calls, I will suggest Vodafone.