Latency is one of VoIP’s biggest enemies. Callers will begin to notice round-trip voice delay of about 250 to 300 ms or greater, which is why latency should be kept under 150 ms one way (source to destination). The further away the SIP provider, the bigger the latency will be.
It’s always a good idea to test the latency to your SIP provider by simply pinging their SIP trunk endpoint. This is a very simple and revealing test that will help you get an indication as to whether latency will be an issue with the provider.