Valuation of Assets and Liabilities other than TPs

We understand that the optionality in the bond or loan should be taken into account when calculating modified duration for the purpose of SCR calculation. How should modified duration be calculated for amortizing loans ("sinkable" repayment profile) ? Those have contractual amortization schedule, but no (not necessarily) contractual call options embedded. Should remaining time to legal maturity be taken into account, without giving consideration to derisking of future exposure via contractually anticipated amortization, or is Weighted Average LIfe ("WAL") to be used as modified duration which prudently shortens the remaining time until legal maturity projecting the exposure decreasing as per contractual schedule until final payment date?