You can also delete messages without having to dial into the normal voicemail system. Unfortunately, UK networks have been slow to adopt Visual Voicemail, and it currently just seems to be O2, EE and Orange that offer it to iPhone users. It’s not available from major networks such as 3 and T-Mobile (even though T-Mobile is now owned by EE).
Phone Call Recorder - ACR is "free." It trumpets that all call recordings, incoming or outgoing, even photo calls—meaning FaceTime calls—are unlimited in length. But that's if you upgrade via in-app purchase to the pro version for $59.99.

While covering all of the relevant information, aim to keep your voicemail to about 20 seconds. You definitely should not ever record a business voicemail longer than 30 seconds, and anything less than 10 seconds will typically mean that you are either speaking too quickly or aren’t providing all of the required information.
“Good afternoon. You have reached the office of [your name]. I will be out of the office beginning on [date] and will be returning on [date]. Please leave a brief message with your contact information, and I’ll be sure to get back to you as soon as I return on [date]. If this is an emergency or you need to speak with someone before I return, please contact [name of colleague/supervisor], [their job title], at [their phone number].”