Hello and welcome to SAYNOTO0870.COM.
I looked through the database and found one possible candidate:
Royal Mail Delivery Office 0845 611 ???? 01285 663553 Cirencester Main Postal Sorting Office
This is presently unverified, so I would like your feedback on whether it's for the right place. If it is, then I will put the 0845 number given in the OP and list it as verified.
On the more general note of the location of the Delivery Office having a different post code for sat navs; that is probably because it has moved out of town.
Google Street View shows a "To Let" sign in the window and
this newspaper article from a couple of years ago talks of a move for the Post Office of 100 yards.
Interestingly, Google Maps and Bing Maps both show GL7 1AA as being on Love Lane Industrial Estate, which suggests that 1AA has moved from the centre of town. However, every sat nav will know where every post code is on its map. But anyone with a sat nav that hasn't been updated since the move will have GL7 1AA as being in Castle Street but (probably) newer sat navs will be programmed with the new location. Just to be sure, they advise that 1YJ is used, which is the actual postcode unit for the location of the delivery office.