Moving from old landlines to VoIP, would that be better for us?

We're a small retail shop in Oxford with 8 staff and 3 phone lines. Our current system is ancient (still on ISDN) and with the 2027 BT switch off looming, I know we have to move to VoIP soon.

INow with the 2027 BT switch off I have to figure this out. I'm considering The VoIP Shop, has anyone used them?

How long are phones down during the switch?

Do we need to buy new desk phones or can we keep using ours?

Anyone lost their number moving over?

We close at 6pm. If I start the switch at 5pm will it be working by morning? That's really what I need to know. Thanks