Microsoft provides MFA at no cost through the implementation of Azure AD security defaults. The only verification option available using this version of MFA is an authenticator application. If a phone call or SMS message is required, then an Azure Active Directory Premium license will need to be purchased. Alternatively, you can utilize a third-party solution to provide MFA for each user in your partner tenant - in this case, it is your responsibility to ensure your MFA solution is being enforced and that you're compliant.