Analysis from elsewhere. Any comments?
TalkTalk - uses their own wholesale / local-loop network
Post Office - uses TalkTalk's wholesale network
iD - is a Mobile MVNO owned & run by TalkTalk not really associated with TalkTalk's wholesale / local-loop network.
However, although there is a common factor - it doesn't from a user point-of-view point towards who is actually at fault. Only verifying call signalling at the four points would identify the exact issue:
TalkTalk core network
TalkTalk interconnection network with Vodafone
Vodafone interconnection network with TalkTalk
Vodafone core network
The reason is that each interconnection is typically treated as a totally different source of call and therefore, the config for manipulation for one destination number may need to be setup separately for each interconnection.
For ring-tone to be received by the call originator - only means that the originator's network has received a specific message from what the originator's network believes is the destination network... who _actually_ creates that specific message can only be identified by troubleshooting the 4 points above.
See the simplified call flow diagram here: https://en.wikipedia.org/wiki/ISDN_User_Part
Therefore, either of the following may apply:
TalkTalk is doing everything correctly and forwards call towards Vodafone - but Vodafone for reasons currently unknown sends back the specific message saying that your number is ringing... which is duly reported as ring-tone to the originating subscribers; however, your number never rings.
TalkTalk thinks you still belong to their network - and when an incoming call to your number arrives - sends to their exchange equipment where they think you are still connected (which is no-longer connected to their exchange equipment as you have switched/ported) which duly rings (but since you are not attached your phone doesn't ring) and returns the specific message saying that your number is ringing.. which is duly reported as ring-tone to the originating subscribers; however, as your phone is no longer connected to TalkTalk's exchange equipment your number never rings.
(The above assumes that in local-loop number switching/porting typical case that your physical connection gets swapped from one land-line operator's exchange equipment to another... though, this may not be entirely true depending on where you live and what equipment the operators have at that location).