Giga $25/mth will be best for you. 1000mins outgoing calls should be more than enough. You can also top up additional mins if you need.
ST Sim only also have promo, $25.35 per month with 1000 mins outgoing calls. It supports VoLTE which other MVNOs do not support. VoLTE is good if you need to call alot, better voice quality. Downside is 12-mth contract.
You can also consider MyRepublic unlimited plan, comes with Unlimited data/sms/mins. If you happen to be MyRepublic broadband customer, it is $38/mth. Otherwise it will be $48/mth.
The VoLTE calls are counted into the Talk Time minutes for both Singtel and M1. Not sure about SH.
https://www.m1.com.sg/support/faq/4g-voice-faq
How much do I pay for 4G Voice?
4G Voice is free for customers on eligible Data Bundled plans with 4G VAS. Calls made using 4G Voice are deducted from the bundled minutes on Data Bundled plans with 4G VAS. Excess talktime will be charged at 18.19 cents per minute for SIM-only Plan and Plan with Device. All other eligible Data Bundled plans will be charged at 16.05 cents per minute.
https://www.singtel.com/business/clearvoice/faq
What are the charges for 4G ClearVoice?
4G ClearVoice is currently free (no monthly subscription) for customers on eligible price plans with 4G VAS.
4G voice minutes are billed at the same rate as your current voice rates. Minutes of usage will be deducted from your price plan bundle and if any, additional talktime Add-Ons. Excess talktime will be charged at the rate of 16.05 cents per minute.
No local data will be deducted from your data allowance for the use of 4G ClearVoice.