A Service Level Agreement (SLA) is the contractually guaranteed level of service from a vendor. For an AI phone assistant four metrics matter: availability (e.g. 99.9 % ≈ 8.8 h downtime per year), response time, recovery time, and live-call latency.
A defensible SLA has two properties: measurable thresholds with a documented measurement method, and consequences on breach — usually service credits or an extraordinary right to terminate. Pure "best effort" clauses are not an SLA.
For business-critical setups (after-hours emergency lines, medical practices, property management) an SLA with ≥99.9 % availability, defined escalation paths and 24/7 support is non-negotiable. Without it, a Sunday-morning outage becomes an operational and reputational incident.