Maybe fttc customers make less phone calls as voip is better. Thus incremental revenue from calls not available.
Voice was same price when fttc had usage limits if I recall.
The VOIP i wouldn't see as a main reason, as they allow override providers , there line rental is basic line rental, not caller ID etc they charge for that,
And i don't doubt that there are a lot of people who have never used Voip on there bb if the increased line rental of £17 or whatever it rises to also included calls then it would be on a par with other providers but £17 for bare line rental just because you also want FTTC , is a bit much
Putting a bit of meat on these figures then your bill would be:
Line Rental Plus: £18.38 [5000 call minutes]. Then there is a line rental price rise coming soon. CLI attracts an additional monthly fee as well.
Unlimited Zen Fibre: £25.00
With VAT on top that's £52.05
At this price level it doesn't exactly put Zen in a competitive price bracket with other ISPs for an equivalent service.
No doubt that Zen's costs have risen over the last year as a justification to increase prices but so have end users base costs risen as well, and these are often not matched with an increase in income. At best income is static and for others a reduction. So if the modest increase in line rental is the straw that breaks the camels back then a cheaper source of supply is sought.
VoIP is a bit of red herring here as setup costs are high and credit accounts attract a monthly fee. A call allowance is often not available either. and the service cannot match the security of POTS. The possibility to actually saving money using VoIP is limited.