In some pairings the CRM provides a framework and the phone vendor provides the app that plugs into it. In others, such as Teams and Dynamics, one company makes both. Each CRM handles this differently, so the first check is whether your two products have a supported pairing.
HubSpot lets you connect a calling app from its App Marketplace. Its help page for third-party calling lists that as available on all products and plans. HubSpot says what happens during and after a call varies by provider. One limit applies whichever provider you use: you can't place a call from a custom object record with a calling app. Where the app supports inbound calls in HubSpot, calls nobody answers are logged as missed calls. If no listed provider fits, HubSpot points developers to its calling SDK.
Salesforce has a date attached. Open CTI is the framework Salesforce has provided for third-party softphones. The Open CTI developer guide now says the framework is in maintenance mode and scheduled for retirement in February 2028. No new features are being added, and Salesforce directs new work to Salesforce Voice. If you're choosing a phone vendor for a Salesforce team, find out what its connector is built on and what the vendor plans to do before that date.
Dynamics 365 has a rule about exclusivity. When the Teams dialer is enabled for a user, other telephony systems connected through the Dynamics Channel Integration Framework stop working for that user. A team can't run the Teams dialer and a second phone system connected through that framework for the same people. The dialer also doesn't work on custom entities or in canvas apps. Microsoft marks call transfer and adding a participant as preview features, which it says aren't meant for production use.
A hosted phone service needs the same check. Atlantech's business phone service page says its hosted voice service integrates with CRM systems, helpdesk software and Microsoft Teams. What you need from any vendor is a supported connector for your exact CRM and edition. Put the CRM and its edition in the quote request, the same way you would list any other requirement when you evaluate UCaaS providers.