Your phone system is important to your business and as such it is recommended that your business keep a PSTN line, at least initially, to ensure customers can still call in and your employees can call out while your VoIP provider. When your VoIP provider is down calls will come in and go out over the PSTN line only. If you have existing PSTN numbers you will probably wish to keep these for advertising purposes until you can update your number with all your contacts and on all advertising material to your new VoIP numbers.