Since you say you are now using BT I suspect your callcenter is located in the UK, is that correct?
There are a number of factors to take in account when selecting a SIP provider:
- the costs for call volume (per minute/second) and subscription costs
- the required amount of concurrent calls (which is linked to the bandwidth required)
- the physical location(s) of your callcenter
- the network connection(s) to your location(s) (capacity / redundancy / QOS)
- the type of phone numbers required(national/international/0800/etc)
- the type of PBX you want to connect the SIP trunk to (Asterisk, Avaya, Axeos, Cisco, etc)
- the SLA(Service Levels) required
You can calculate what your cost savings will be when changing your provider, based on the CDR information/statistics you will gather in the upcoming year.