It varies enormously, and cheap providers are cheap for a reason, usually gray routes that get reordered every week. Test before committing, and test properly: call from multiple local mobile carriers, not just one, at different times of day, and record the calls so you can compare rather than rely on memory. Listen for one way audio and post dial delay, those are the usual tells. Ask the provider directly whether routes are direct or wholesale. We landed on HotTelecom https://hottelecom.biz/virtual-number-of-azerbaijan.html , after testing three vendors, and quality has held. Also check your own upstream bandwidth before blaming anyone, that has burned me too. Also ask what happens during an outage, whether there is a failover route or your calls simply stop landing.