Change keypad to letters

Using the phone itself and its keypad will not produce letters, and neither would your home phone, if you still have or had one. I cannot understand how a company would expect you to enter letters into the phone for a code. I suspect as soon as you have tapped on the keypad of the phone the one time, it is only sending the digital signal for that number and cannot change to anything else.

iPhone call keypad -> each number from 2-9 corresponds to 3 or 4 letters. 

That number can be any of those 3 or 4 letters, so you could press the 2 that corresponds to either an A , B  or C. 

For example if you dialing 1-800-555-APPL would be 1-800-555-2775  

Hope this works for you.

Glad you got it sorted. I remember the days of texting that way, and also remember what was called T9 text when you were trying to find contacts on phones. That is strange that they expected you to be able to enter alpha characters on the phone.

See this Apple article: Switch apps on your iPhone, iPad or iPod touch

https://support.apple.com/en-gb/HT202070