Let's improve this code (in two different ways.)

refactoring101.cs

// Original code - let's improve it...
public bool IsTelephoneNumberInUse(int clientId, int telephoneId)
{
    ISubscriptionService service = GetService<ISubscriptionService>();
    IList<Subscription> subs = service.GetSubscriptions(clientId, false);
    bool inUse = false;

    foreach (Subscription sub in subs) {
        if (sub.ContactPhoneId == telephoneId) {
            inUse = true;
            break;
        }
    }

    return inUse;
}

// First revision (assuming you don't know about LINQ or can't use it for some reason.)
public bool IsTelephoneNumberInUse(int clientId, int telephoneId)
{
    ISubscriptionService service = GetService<ISubscriptionService>();
    IList<Subscription> subs = service.GetSubscriptions(clientId, false);

    foreach (Subscription sub in subs) {
        if (sub.ContactPhoneId == telephoneId) {
            return true; // Using a break inside of this block is absolutely pointless, just return instead.
        }
    }

    return false; // By simply returning a default, we can remove the pointless 'inUse' variable.
}

// Second revision - the LINQ approach.
public bool IsTelephoneNumberInUse(int clientId, int telephoneId)
{
    ISubscriptionService service = GetService<ISubscriptionService>();
    IList<Subscription> subs = service.GetSubscriptions(clientId, false);

    return subs.Any(x => x.ContactPhoneId == telephoneId); // Stop being an idiot, and start using System.Linq.EnumerableExtensions!
}