I use Flextel myself. Where possible, with a lot of withheld numbers that have called me anyway, it will tell you the number that called except for the last three digits. For example, 01234 567xxx. It will also offer the option to block this number as they know the last three digits. That's the primary reason I use it, it can come in handy.

Flextel also have an IVR facility, record your own message and configure it as you want. I use the IVR for managing automated/spam calls so they don't easily get through to me.

However, that said while there's typically no monthly fee it might not be as cheap as Sipgate Basic depending on the call volume. There are monthly plans which give you bundled minutes though. Outside of the monthly plans, call rates are higher than Sipgate basic, unless you have a monthly plan with Flextel then I believe it's 5p/min + 10p setup - at least to local numbers. You will get charged for incoming calls that you answer if they go to a SIP/VoIP destination instead of to an actual phone number, same per minute charge applies as if it were an outgoing call just no setup I believe.

There's no line rental as such, but I recall somewhere I'm sure they said they may charge a small fee if the number has had no activity for quite some time (possibly 180 days or more), effectively as some kind of 'renewal'. Not sure if that's still the case.

As for porting numbers, that should be fine too. You need to complete an 'importing' form. Buying numbers can be cheap (as low as £1 one-time fee for local), you can choose the number you want from a list of them.

Voipfone have a call blocking option. I don't use. I am registered with the Telephone Preference Service. I only get a few spam calls on the Voipfone line as the number is one I ported from BT. I also have two Sipgate and one A&A Voip lines which never get spam calls.