I made the sad decision to work for genie - BT's mobile phone service although you were never meant to tell customers that it was BT !, for about 18months.
I have so many upsets and problems with BT I could literally write a book! (and by all means quiz me on this !

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However I will tell you this .... whenever writing to BT and you want action- real action - ALWAYS copy t to watchdog and to OFCOM, then and only then will they start to listen - they literally ignore all others as they have SO many complaints ! - Well genie did anyway ....
Although I sadly have to keep a BT line I use Virgin and although most of their service has been excellent - they did mess up badly when I was going N to S and had to deal with a mess - yes that BT started !
I had had Onetel and it worked fine - but it made no sense to keep both Virgin (B/Band) and Ontel the 'all calls fro one fee' so I dropped the Ontel - well BT's screw up had disconnected everyone anyway ! So I just redid a deal with Virgin, and they have given me a very good monthly deal.
Sadly their size is making a sadder call centre attitude, but they are nothing like BT thanks goodness.
I use BT landines (2) and then have nothing extra on one ( and BE CAREFUL of BT's attempts to get you back ! - the small print shows you are on a rolling 12 mth contract etc), and on the other line which includes 'call sign' (the reason why I cannot change away from the BT line) I then have all calls via Virgin for a one off fee.
I will move the Safe Speed line away from BT when that gets cheap enough and I have complete the next 12 month contract !
Be aware too - if you use a non BT line provider if you ever call 999 your details will NOT automatically appear on their screens, so if or any reason you cannot say much they may not know where you are. It is a small 'point' but .....