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).
If you recently restored or updated your iPhone, your network setting may have been affected and may be causing “iPhone voicemail error try again later.” Please reset your network settings. Go to Settings > General > Reset > Reset Network Settings Enter your password. Press Reset network setting in red font. The phone will automatically reset it. After it restarts, your phone then tries voicemail again.

6 Do’s and Don’t’s of writing Voicemail Recording Scripts Follows the do’s and don’t of leaving the correct voicemail for your clients. Write a new voicemail greeting for office following these 6 principles. A lawyer’s office needs to maintain professionalism at all times in order to record the perfect voicemail greeting. Let’s look at an voicemail greetings example.
20. "Hey there, this is [your name] from [your company]. I'm out of the office until [date]. In the meantime, please direct your inquiries to [coworker's name] at [email address]. They can also be reached at [phone number]. Thank you."