Hi rdbb1831, the method for increasing the number of rings can vary depending on the phone you have.
For CDMA phones (no SIM card), dial *94 + 5 + SEND
For HSPA/LTE phones (with SIM), dial *004* <Voicemail Access Number> ** <Timer> # + SEND.
Or *004* <Voicemail Access Number> ** <Timer> # + SEND.
- The Timer # must be set in increments of 5 to a maximum of 30 seconds.
You will need to call in to obtain your Voicemail Access Number