An update (I was a little wrong in the timeframe, good job I checked!). Our contract actually expired 2 months before I thought it did.
We gave ourselves a 2 week window, booking Zzoomm to install 14 days prior to the Plusnet contract expiring. As soon as it was installed, and working, we signed up to A&A. There were a few questions they asked via email, mainly revolving around whether it was a single number or a block that all needed porting. Basically told them it's a single residential number from Plusnet with FTTC service, confirmed the address and losing supplier, and they worked their magic. The number then ported over 6 days later (although they said they needed 10), just prior to our PN contract end date. Really easy process, and no dramas. The phone line was completely dead (would not ring when rung from a mobile) for approx 30 mins from when the PN router went offline, then routed through to my SPA112 fine.
£15 transfer fee later, and for the princely sum* of £1.44 a month, we have our old landline number on a VOIP provider, with our old landline working with it through a Cisco SPA112. Boom!
* Tongue in cheek, consider some companies were quoting upwards of £10 a month for transferring and using your landline, this is a fantastic price!
In Addendum: Interestingly enough, the Adtran SDX622V ONT Zzoomm installed has some BT431A FXS ports, although obviously they appear that they can't be configured by the end user... which is a shame as it may avoid me having to use my SPA112 if it could route out over the internet!
