def volatilityCall (S, K, T, t, q, r, C_true):

    sigma = sigmaHat(S, K, r, q, T, t)
    CVega = callOrPutVega(S, K, sigma, r, q, T, t)

    call_Value = callValue(S,K,sigma,r,q,T,t)

    tol = 1e-8
    sigmaDiff = 1
    n = 1
    nmax = 100

    while (sigmaDiff >= tol and n < nmax) :
        increment = (call_Value-C_true) / CVega
        sigma = sigma - increment
        n = n+1
        sigmaDiff = abs(increment)

        call_Value = callValue(S, K, sigma, r, q, T, t)
        CVega = callOrPutVega(S, K, sigma, r, q, T, t)

    return sigma