If they are in fact using VoIP, and there is a solution, it will be out of your hands. First thing I would do is try to get them to verify that they are using VoIP. I would also try to confirm that latency was indeed significant by doing a loop back test. A simple loopback would involve making 2 calls to the site in question from another site that is not on VoIP. Set up 2 phones side by side at the site in question. Make 2 connections to the remote site and then conference the calls at the remote site. Speak into one of the phones while listening on the other. The delay you hear, if any, will be the latency. If you can hear a delay then latency is going to be a problem.
At this point, provided you have audible latency, there are several things you might want to consider.
1) Get the provider to address the latency.
2) Inform and demonstrate the delay to the customer. There is no incentive for the provider to listen to you, they will listen to the customer.
3) If the present provider is unwilling, or unable, to address the problem consider switching providers.
I realize there can be a lot of politics involved here. The provider is going to tell the customer that it isn't their problem. You have to show the customer that it is indeed their problem. With the customer on your side you will have a lot more leverage to get the problem addressed.
