Possible solution to retry calls and escalate when the callee does not respond within a certain time.

RetryCallsAndEscalate.cs

namespace DurableFunctions.Demo.DotNetCore.RetryCallsWithBackup
{
    public class EscalatingCallsOrchestrator
    {
        [FunctionName(nameof(RunEscalatingCallsOrchestrator))]
        public async Task RunEscalatingCallsOrchestrator(
          [OrchestrationTrigger] IDurableOrchestrationContext context,
          ILogger logger)
        {
            var escalatingCallsInput = context.GetInput<EscalatingCallsInput>();

            var resultCall1 = await context.CallSubOrchestratorAsync<bool>(
                nameof(RunRetryCallsOrchestrator),
                escalatingCallsInput.PhoneNumbers[0]);
            
            if (!resultCall1)
            {
                // Call1 has not been received within the wait time. Let's try Call3.
                var resultCall2 = await context.CallSubOrchestratorAsync<bool>(
                    nameof(RunRetryCallsOrchestrator),
                    escalatingCallsInput.PhoneNumbers[1]);

                if (!resultCall2)
                {
                    // Call2 has not been received within the wait time. Let's try Call3.
                    var resultCall3 = await context.CallSubOrchestratorAsync<bool>(
                        nameof(RunRetryCallsOrchestrator),
                        escalatingCallsInput.PhoneNumbers[0]);
                }
            }
        }

        [FunctionName(nameof(RunRetryCallsOrchestrator))]
        public async Task<bool> RunRetryCallsOrchestrator(
          [OrchestrationTrigger] IDurableOrchestrationContext context,
          ILogger logger)
        {
            var input = context.GetInput<RetryCallInput>();
            var maxWaitTimeBetweenCalls = TimeSpan.FromMinutes(5/3); // max 5 min wait time, divided by 3 attempts to call
            await context.CallActivityAsync("MakeCall", input.PhoneNumber);
            // Orchestrator will wait until Call event is received or maxWaitTimeBetweenCalls is passed, defauls to false.
            var resultCall = await context.WaitForExternalEvent<bool>("Call", maxWaitTimeBetweenCalls, false);
            if (input.RunCount < 2)
            {
                input.RunCount += 1;
                context.ContinueAsNew(input);
            }

            return resultCall;
        }

    }

    public class EscalatingCallsInput
    {
        public string[] PhoneNumbers { get; set; }
    }

    public class RetryCallInput
    {
        public string PhoneNumber { get; set; }

        public int RunCount { get; set; }
    }
}