The latter isn't really necessary as the number isn't given out.
I guess you were on Simply but are now on Complete, then?
I am with AAISP at the moment, just looking to save a little money as priorities have changed and £60 is a lot for just an Internet connection these days!
Yes the Talk2Go app uses your smartphone 3G/4G or wifi data, you have the option to use wifi data only, but TBH data usage of the app is in KB's rather than MB's so i leave both on. Any calls you make from your linked smartphone show up as your mobile number and you have the option to receive landline calls on your mobile, though I have this switched off.
I'm currently on Complete package, currently paying around £38 pm for everything (line rental included).
TalkTalk Business 80/20
Netgear R9000 X10 running OpenWRT
My Broadband Quality Monitor