Extension numbering and extension length is important and deserves some thought before installation. You need to consider growth, expansion capacity always remembering that you'll need extensions for hut groups, queues, conferencing, IVR's and so on.
3CX for example requires yo to select extension length at installation time and if you want to change a full re-installation is required.
There's a few other things to consider especially if you choose three digit extensions. In the UK three digit numbers beginning 1 are used for services such as operator (100), police non emergency (101), NHS Helpline (111) not forgetting 112 the EU wide emergency number.
If you choose three digit extension starting with 1 you'll not be able to dial any of these numbers. If you do choose three digit extension numbers we suggest therefore you start at 200.