thats all you need..no need for a landline...who uses landlines these days?hardly anyone is the answer!
Older customers, who might be able to use the interweb, but then find it easier to punch six numbers into a phone, rather than eleven.
Older customers, the ones with the grown up kids, mortgage paid, and too lazy or decrepit to clean their own windows.
Older customers; the ones who remember to ensure that you're paid; the ones who feel a bit 'thing' owing money.
Those ones still prefer a landline.