How To Set Up Your Own VoIP System At Home - Ars Technica

So, I created an extension with no phone attached, for whitelisting. In 3CX, you can specify the destination for an incoming number to be a particular extension, a ring group, an attendant, and so on. Calls to the XD number go to the whitelist extension. On that extension, the forwarding rules tab has an exceptions list, which can have up to 15 entries. That covers the numbers of family members, whose calls are connected directly to the VIP ring group. Other calls get the auto-attendant, and have to do the maths. You can also direct anonymous calls direct to voicemail, if you like.

Whatever sort of rules you set up, test them thoroughly. Call in to your temporary number from a mobile; check calls work in both directions. You may need to open certain ports on your firewall for incoming SIP traffic—usually the provider will be able to give you a list of which ones they need.

One of the reasons it’s important to check this before you switch a main number over to VoIP is that once you make the switch, you can’t go back easily. Porting the number from a BT line implicitly ceases the service on that line. So, moving the block of 10 numbers from my ISDN also cancelled it. If you port a number that has DSL service, the line is cut off, and with it the DSL.

So how do you keep your existing number and make the switch to VoIP? If there’s no DSL on the line, just do it. If there is DSL, you have two alternatives—and neither are convenient. You could have a new line connected for the DSL, with a different number, which costs a fair bit. Or you could have the old line reactivated after the number porting, which will give it a new number. This is cheaper, but you’ll be without broadband for a few days. You’d also be without phones, unless your VoIP provider sets up temporary call forwarding while you wait for the broadband to be restored.