If you just want a VOIP line for the occasional incoming call (handy for "grandma" and the likes who may not have a mobile) AAISP will rent you a VOIP line for £1 (+VAT) ie £1.20 pm. No inclusive calls and outbound charges are per minute
I had a FTTC connection with landline phone via PlusNet and separately had FTTP installed from Zen. Once I was happy with FTTP I ported my landline number from PlusNet to AAISP (which happily had the effect of ceasing my PlusNet FTTC connection).
AAISP charge £15 to port your number in
see here
This ISP Preview article is informative about
porting
The standard supplied Zen Router (Fritzbox 7530) has the ability to connect to a VOIP provider (or you can purchase your own ATA adapter)