I only use VOIP for incoming calls as we've a good mobile package so it's better to use the minutes on the mobiles to call out.
I've had a few Sipgate accounts for several years, never had a problem with them. I use CSipSimple on my mobile for those and can have several numbers active at once. Line quality is usually clear unless I'm in a poor signal area. I have one, maybe two, of those lines that have credit for outgoing but have only used that for testing or pranking friends
I have a paid for A&A number too, think it's about £1.20 a month ? I use a Camrivox ATA connected to some Panasonic DECT phones and that's my main home line. It's a proper UK number for the town I live in, not just SIP. Only a problem if the Internet goes off.
I've used the Sipgate numbers on the Camrivox too.
As long as you set them up right they should be fine. I've not needed to port numbers to VOIP. Happy to get rid of a line that got lots of junk calls last year when I decided to go VOIP only.
Virgin Cable (200/12) + EE Mobile BB