We eventually decided that our existing PABX solution no longer met requirements. In particular, it was very opaque, and extremely expensive, with a licensing model (and vendor locked-in handset requirements) that were punishingly expensive.
We considered "DIY" with something from Yealink (most likely an S300), with an interim set of BRI and POTs links to move away from Avaya onto another platform, before investigating SIP trunks for the "uplink", but we looked at our work schedules (and the offers that came in) and we ultimately decided to get a VOIP telecommunications company to help us install it, implementing SIP from launch.
Here are a few things we learned along the way...
