How to increase ring numbers for answer phone?

[#1]

I have been trying to do this but it's not working. I have an Apple Iphone 6. Its dial pad does not have a + sign.

Someone please explain how to increase rings to 30 rings. I am in contact with hospitals asnd they do not leave a ring back message or a number and do not give me enough time to reach the phone

[#2]

The GSM code for changing the ringtone length is...

**61*[voicemail number]*11*[length of delay]#

On EE, you can choose 5 second intervals between 5-30 seconds, whether it's the same on PN is anyone's guess. Same core network, but different services (voicemail, shortcodes, VoLTE/WiFi-calling)...