Can't you say something like "We apologize for not answering your call [and for the inconvenience we have caused]" (even leaving off the last part?)?
مرسىىىىىىىىىىىىىىىىىىىىىى ىىىىىىىىىىىىىىىىىىىىىىىىى ىىىىىىىىىىىىىىىىىىىىىىىىى ىىىىىىىىىىىىىىىىىىىىىىىىى ىىى