I'm currently doing the same on my organization.
The first thing to know is that there is two ways to use Skype for Business as the phone system:
- Using PBX Connector to join existing On Premises telephony system.
- Using Cloud PBX and a PSTN Calling Plan.
We were using sip lines hosted directly on our provider by the past, so as we never had on premises equipments regarding the Telephony system we decided to switch on the Cloud PBX solution.
- Everything is hosted on Azure, and is managed directly from Skype for Business admin portal or Office 365 Powershell cmdlets.
- It costs a lot, but enables us to have a work number from everywhere, without having to add extra costs for mobile telephony solutions.
For the pricing, this is simple, if you want to enable calling on skype for business you will need to buy two extra licences:
- Skype for Business Cloud PBX - 6.70€/month/user (french prices here)
- PSTN calling Plan: 10,10€/month/user for the National calling Plan, 20.20€/month/user for the International plan. The two plans are including 1000 minutes of communications.
Now, if you have Office 365 E5 Subscriptions CloudPBX is included into the Subscription, and you'll only have to take the calling plan. (And you'll have the PSTN Conferencing options which is nice too)
You can as well transfer old numbers into Skype for Business, but it take 2 weeks approx.
Kind Regards,