The key when calling a number beginning 0300 is to ensure that you are on the correct calling plan in the first place
If you have inclusive calls to 01 and 02 numbers, this will also cover calls to 03 numbers.
If you are paying a per-minute rate for calls to 01, 02 and 03 numbers you’re probably on the wrong call plan.
Most people make calls to 01, 02 and 03 numbers as part of an inclusive allowance. Outside of that allowance, landline providers charge up to 12p per minute, e.g. weekday calls made when only a “weekend” plan is in force. Mobile providers charge up to 45p per minute, e.g. exceeding a contract’s monthly call allowance or pay-as-you-go without a call bundle.
It only takes about 45 to 60 minutes of calls per month on a landline with a “weekend” deal for the “anytime” deal to work out cheaper.
It only takes about 10 to 20 minutes of calls per month on a mobile paying a per-minute rate for the next contact up in size (e.g. almost double the minute for less than a fiver extra) or a pay-as-you-go call bundle (e.g. 100 minutes for £5) to work out cheaper.