Many of the virtues of fasting and its reward are not just limited to Ramadan except it is obviously stated that it stipulated to Ramadan alone, therefore those who are used to fasting voluntary fast are far far far ahead of those who just fast Ramadan only, they have carried all the reward that is acquired by fasting.
The following hadith is not related to Ramadan alone it refers to other voluntary fast as well:

The Prophet (blessings and peace of Allah be upon him) said: “Allah, may He be glorified and exalted, said: ‘Every deed of the son of Adam is for him, except fasting. It is for Me and I shall reward for it.’” Narrated by al-Bukhaari (1904) and Muslim (1151). 

Fasting has no equal (al-Nisaa'i, 4/165; Saheeh al-Targheeb, 1/413), and the du’aa’ of the fasting person will not be refused (reported by al-Bayhaqi, 3/345; al-Silsilat al-Saheeh, 1797).
Fasting will intercede for a person on the Day of Judgement, and will say, “O Lord, I prevented him from his food and physical desires during the day, so let me intercede for him.” (Reported by Ahmad, 2/174. Al-Haythami classed its isnaad as hasan in al-Majma’, 3/181. See also Saheeh al-Targheeb, 1/411). The smell that comes from the mouth of a fasting person is better with Allaah than the scent of musk. (Muslim, 2/807). Fasting is a protection and a strong fortress that keeps a person safe from the Fire. (Reported by Ahmad, 2/402; Saheeh al-Targheeb, 1/411; Saheeh al-Jaami’, 3880). Whoever fasts one day for the sake of Allaah, Allaah will remove his face seventy years’ distance from the Fire. (Reported by Muslim, 2/808). Whoever fasts one day seeking the pleasure of Allaah, if that is the last day of his life, he will enter Paradise. (Reported by Ahmad, 5/391; Saheeh al-Targheeb, 1/412).


For those who are not used to fasting voluntary fast see what you are missing.
They fast voluntary fast now they are fasting Ramadan, what a bounty.