Hi, Only just realised this was still up.
All has been set up and I'm now enjoying my Asterisk on RasPI running multiple lines including one for spam calls from BT line about PPI that go nowhere so I'm never bothered with automated calls. If anyone is interested I have made a video on YouTube highlighting who its done. I use it for biz and home and my annual call level is ~5,000 calls/pa, call cost ~£40/pa.
https://youtu.be/2dmm_XHpvKI
I have also finished an enterprise installation for a corporate office in Lincolnshire so if you are interested but have no skill my IT dpt may take the job on. Installation featured on YT is recommended for small business so instead of giving out your mobile out to clients, you will instead give out proper landline number with greetings and recordings like your utility company etc... and have it connected with your mobile. (No costly call redirect)