Re: VoIP is fine...
"You always still have a PSTN gateway, but you move as many people off it as you can. A slightly higher internet connection is going to be cheaper than multiple landlines."
Out here, internet connections come in two flavours: Expensive or Unreliable
Unreliable is the current ADSL link. Approx 6mbit, so not the worst around, but the local exchange, despite serving a sparsely populated area, regularly suffers contention. Add to that past experience of losing ADSL for two days versus an ISDN line that only seems to die if we suffer direct lightning strikes and ISDN wins on unreliable.
Yes we could go for a leased line, but a 2mbit (burstable to 10mbit) leased line here costs over £1,700 a month (no, that isn't a typo). Our 12 channel ISDN30 is around £170 a month so is a no-brainer for us.
We've deployed VoIP/SIP trunks at other offices (usually those with FTTC) and it works well. If we ever get FTTC/FTTP service to our main office then I will probably bin the ISDN30, but until then it can keep solidering on.